Not true, and also not relevant. Any 68k CPU with 32bit addressing space can deal with that ammount of RAM, given that the rest of the hardware permits it.
It is not relevant for the discussion where you put that RAM. Someone could make a CPU card for A3000/A4000 with 2GB RAM on board, it is technically doable and AmigaOS would support it.
And my old minimac with 16GB RAM runs in circles around your G5, especially if you are running MorphOS that only supports one core and is limited to 2GB(?) of RAM, just like AmigaOS is. Which was my point.
Would I be correct assuming that someone could do a 32 bit Microprocessor CPU with 2 Gb that AmigaOS would run on that?