You found a problem in the API and I opened a GitHub issue here. You can find more details on why it's not working in said issue.

The bottom line is that this object is indeed missing tertiary node implementation and therefore cannot be connected to the generator nodes.

So you have two options:

• Use a ChillerAbsorptionIndirect object. I strongly suggest you do that. The I/O ref is quite explicit here: ChillerAbsorptionIndirect is exactly the same as ChillerAbsorption but better, and ChillerAbsorption may get deprecated as a result... And the good thing is that ChillerAbsorptionIndirect does allow connecting the generator nodes.
• Indeed create a hot water loop with a pkaceholder component boiler on the demand supply side, and after FT you switch the component boiler with the Chiller:Absorption and make sure you update the Branches etc (or you could also not use a component at all and completely add a new branch but that tends to be harder)etc.