There's a very big difference between installing OS 9 on the G4's that did not support OS 9, and installing it on a G5. First up, the G4's were still using essentially the same motherboard for quite a while, which had just minor updates done to it.
The G5 on the other hand is (at a minimum, there's plenty more):
- A completely new processor
- A completely new motherboard design
- A new type of hard disk
- A new way of connecting most of the system peripherals on the motherboard
In short, OS 9 isn't going to be booting natively on a G5 without someone writing drivers for these things.
- proton