"However, the maximum power output of the generator decreases with altitude."
If the APU runs on constant RPM it simply means the gas turbine has enough power for the generator.
Yep, at some point the turbine may give up, but the generator doesn't "lose power with altitude" it will trip off.
Look at the #2 graph of Smythe. 90kVA all the way....
The "Generator" referred to in the OP's question is the combination of APU gas turbine + Alternator. (i.e. ability to generate electricity)
"90 KVA all the way ...." depends on ISA deviation (for the gas turbine output). If the Alternator isn't required to deliver 90 KVA (say 45 KVA) then the graph would show ability for the Generator to provide that power to higher FLs.