See the EnergyPlus IO Ref which says...

Coil:Cooling:DX:MultiSpeed

...Currently, this cooling coil can only be referenced by a AirLoopHVAC:UnitarySystem or AirLoopHVAC:UnitaryHeatPump:AirToAir:Multispeed object....

The OpenStudio API is behaving correctly. Note that trying to add a CoilCoolingDXMultiSpeed to the air loop returns false, but returns true for the CoilCoolingDXVariableSpeed and FanVariableVolume allowed classes.

PS C:\Users\matt> irb
irb(main):001:0> require 'openstudio'
=> true
irb(main):002:0> include OpenStudio::Model
=> Object
irb(main):003:0> m = Model.new
=> #<OpenStudio::Model::Model:0x00000002c045e8 @__swigtype__="_p_openstudio__model__Model">
irb(main):004:0> al = AirLoopHVAC.new(m)
=> #<OpenStudio::Model::AirLoopHVAC:0x000000064dbcb8 @__swigtype__="_p_openstudio__model__AirLoopHVAC">
irb(main):005:0> cc_dx_vs = CoilCoolingDXVariableSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXVariableSpeed:0x0000000685b4d8 @__swigtype__="_p_openstudio__model__CoilCoolingDXVariableSpeed">
irb(main):006:0> cc_dx_ms = CoilCoolingDXMultiSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXMultiSpeed:0x00000006743190 @__swigtype__="_p_openstudio__model__CoilCoolingDXMultiSpeed">
irb(main):007:0> al_so_node = al.supplyOutletNode
=> #<OpenStudio::Model::Node:0x0000000686aac8 @__swigtype__="_p_openstudio__model__Node">
irb(main):008:0> cc_dx_vs.addToNode(al_so_node)
=> true
irb(main):009:0> cc_dx_ms.addToNode(al_so_node)
=> false
irb(main):010:0> f = FanVariableVolume.new(m)
=> #<OpenStudio::Model::FanVariableVolume:0x000000068d9400 @__swigtype__="_p_openstudio__model__FanVariableVolume">
irb(main):011:0> f.addToNode(al_so_node)
=> true
irb(main):012:0>

See the EnergyPlus IO Ref which says...

Coil:Cooling:DX:MultiSpeed

...Currently, this cooling coil can only be referenced by a AirLoopHVAC:UnitarySystem or AirLoopHVAC:UnitaryHeatPump:AirToAir:Multispeed object....

The OpenStudio API is behaving correctly. Note that trying to add a CoilCoolingDXMultiSpeed to the air loop returns false, but returns true for other allowed classes.

PS C:\Users\matt> irb
irb(main):001:0> require 'openstudio'
=> true
irb(main):002:0> include OpenStudio::Model
=> Object
irb(main):003:0> m = Model.new
=> #<OpenStudio::Model::Model:0x00000002c045e8 @__swigtype__="_p_openstudio__model__Model">
irb(main):004:0> al = AirLoopHVAC.new(m)
=> #<OpenStudio::Model::AirLoopHVAC:0x000000064dbcb8 @__swigtype__="_p_openstudio__model__AirLoopHVAC">
irb(main):005:0> cc_dx_vs = CoilCoolingDXVariableSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXVariableSpeed:0x0000000685b4d8 @__swigtype__="_p_openstudio__model__CoilCoolingDXVariableSpeed">
irb(main):006:0> cc_dx_ms = CoilCoolingDXMultiSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXMultiSpeed:0x00000006743190 @__swigtype__="_p_openstudio__model__CoilCoolingDXMultiSpeed">
irb(main):007:0> al_so_node = al.supplyOutletNode
=> #<OpenStudio::Model::Node:0x0000000686aac8 @__swigtype__="_p_openstudio__model__Node">
irb(main):008:0> cc_dx_vs.addToNode(al_so_node)
=> true
irb(main):009:0> cc_dx_ms.addToNode(al_so_node)
=> false
irb(main):010:0> f = FanVariableVolume.new(m)
=> #<OpenStudio::Model::FanVariableVolume:0x000000068d9400 @__swigtype__="_p_openstudio__model__FanVariableVolume">
irb(main):011:0> f.addToNode(al_so_node)
=> true
irb(main):012:0>

See the EnergyPlus IO Ref which says...

Coil:Cooling:DX:MultiSpeed

...Currently, this cooling coil can only be referenced by a AirLoopHVAC:UnitarySystem or AirLoopHVAC:UnitaryHeatPump:AirToAir:Multispeed object....

The OpenStudio API is behaving correctly. Note that trying to add a CoilCoolingDXMultiSpeed to the air loop returns false, false, but returns true for other allowed classes.

PS C:\Users\matt> irb
irb(main):001:0> require 'openstudio'
=> true
irb(main):002:0> include OpenStudio::Model
=> Object
irb(main):003:0> m = Model.new
=> #<OpenStudio::Model::Model:0x00000002c045e8 @__swigtype__="_p_openstudio__model__Model">
irb(main):004:0> al = AirLoopHVAC.new(m)
=> #<OpenStudio::Model::AirLoopHVAC:0x000000064dbcb8 @__swigtype__="_p_openstudio__model__AirLoopHVAC">
irb(main):005:0> cc_dx_vs = CoilCoolingDXVariableSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXVariableSpeed:0x0000000685b4d8 @__swigtype__="_p_openstudio__model__CoilCoolingDXVariableSpeed">
irb(main):006:0> cc_dx_ms = CoilCoolingDXMultiSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXMultiSpeed:0x00000006743190 @__swigtype__="_p_openstudio__model__CoilCoolingDXMultiSpeed">
irb(main):007:0> al_so_node = al.supplyOutletNode
=> #<OpenStudio::Model::Node:0x0000000686aac8 @__swigtype__="_p_openstudio__model__Node">
irb(main):008:0> cc_dx_vs.addToNode(al_so_node)
=> true
irb(main):009:0> cc_dx_ms.addToNode(al_so_node)
=> false
irb(main):010:0> f = FanVariableVolume.new(m)
=> #<OpenStudio::Model::FanVariableVolume:0x000000068d9400 @__swigtype__="_p_openstudio__model__FanVariableVolume">
irb(main):011:0> f.addToNode(al_so_node)
=> true
irb(main):012:0>

See the EnergyPlus IO Ref which says...

Coil:Cooling:DX:MultiSpeed

...Currently, this cooling coil can only be referenced by a AirLoopHVAC:UnitarySystem or AirLoopHVAC:UnitaryHeatPump:AirToAir:Multispeed object....

The OpenStudio API is behaving correctly. Note that trying to add a CoilCoolingDXMultiSpeed to the air loop returns false, but returns true for other allowed classes.correctly.

PS C:\Users\matt> irb
irb(main):001:0> require 'openstudio'
=> true
irb(main):002:0> include OpenStudio::Model
=> Object
irb(main):003:0> m = Model.new
=> #<OpenStudio::Model::Model:0x00000002c045e8 @__swigtype__="_p_openstudio__model__Model">
irb(main):004:0> al = AirLoopHVAC.new(m)
=> #<OpenStudio::Model::AirLoopHVAC:0x000000064dbcb8 @__swigtype__="_p_openstudio__model__AirLoopHVAC">
irb(main):005:0> cc_dx_vs = CoilCoolingDXVariableSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXVariableSpeed:0x0000000685b4d8 @__swigtype__="_p_openstudio__model__CoilCoolingDXVariableSpeed">
irb(main):006:0> cc_dx_ms = CoilCoolingDXMultiSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXMultiSpeed:0x00000006743190 @__swigtype__="_p_openstudio__model__CoilCoolingDXMultiSpeed">
irb(main):007:0> al_so_node = al.supplyOutletNode
=> #<OpenStudio::Model::Node:0x0000000686aac8 @__swigtype__="_p_openstudio__model__Node">
irb(main):008:0> cc_dx_vs.addToNode(al_so_node)
=> true
irb(main):009:0> cc_dx_ms.addToNode(al_so_node)
=> false
irb(main):010:0> f = FanVariableVolume.new(m)
=> #<OpenStudio::Model::FanVariableVolume:0x000000068d9400 @__swigtype__="_p_openstudio__model__FanVariableVolume">
irb(main):011:0> f.addToNode(al_so_node)
=> true
irb(main):012:0>

See the EnergyPlus IO Ref which says...

Coil:Cooling:DX:MultiSpeed

...Currently, this cooling coil can only be referenced by a AirLoopHVAC:UnitarySystem or AirLoopHVAC:UnitaryHeatPump:AirToAir:Multispeed object....

PS C:\Users\matt> irb
irb(main):001:0> require 'openstudio'
=> true
irb(main):002:0> include OpenStudio::Model
=> Object
irb(main):003:0> m = Model.new
=> #<OpenStudio::Model::Model:0x00000002c045e8 @__swigtype__="_p_openstudio__model__Model">
irb(main):004:0> al = AirLoopHVAC.new(m)
=> #<OpenStudio::Model::AirLoopHVAC:0x000000064dbcb8 @__swigtype__="_p_openstudio__model__AirLoopHVAC">
irb(main):005:0> cc_dx_vs = CoilCoolingDXVariableSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXVariableSpeed:0x0000000685b4d8 @__swigtype__="_p_openstudio__model__CoilCoolingDXVariableSpeed">
irb(main):006:0> cc_dx_ms = CoilCoolingDXMultiSpeed.new(m)
=> #<OpenStudio::Model::CoilCoolingDXMultiSpeed:0x00000006743190 @__swigtype__="_p_openstudio__model__CoilCoolingDXMultiSpeed">
irb(main):007:0> al_so_node = al.supplyOutletNode
=> #<OpenStudio::Model::Node:0x0000000686aac8 @__swigtype__="_p_openstudio__model__Node">
irb(main):008:0> cc_dx_vs.addToNode(al_so_node)
=> true
irb(main):009:0> cc_dx_ms.addToNode(al_so_node)
=> false
irb(main):010:0> f = FanVariableVolume.new(m)
=> #<OpenStudio::Model::FanVariableVolume:0x000000068d9400 @__swigtype__="_p_openstudio__model__FanVariableVolume">
irb(main):011:0> f.addToNode(al_so_node)
=> true
irb(main):012:0>

See the EnergyPlus IO Ref which says...

Coil:Cooling:DX:MultiSpeed__Coil:Cooling:DX:MultiSpeed__

...Currently, this cooling coil can only be referenced by a AirLoopHVAC:UnitarySystem or AirLoopHVAC:UnitaryHeatPump:AirToAir:Multispeed object....