That's complete bunk.

I've got disks way bigger than that with the OS installed entirely on the second partition.

The OS puts data in the first few blocks of the disk that tells the ROM where to find the rest of the system data. The Mac's ROM contains code to access all the disk space addressable by the controller, so once the ROM takes over at boot from Open Firmware, it doesn't matter where the OS is installed. As OS X doesn't use the on-board ROM, things are much trickier. Hence the 8 GB partition restriction.

Peace,

Drew