I can't find anything claiming this. This is literally the first time I have ever seen this claim. The original fat PS2 including majority of the PS1 hardware

I can't find anything claiming this. This is literally the first time I have ever seen this claim. The original fat PS2 including majority of the PS1 hardware physically in the machine. More and more of it got offloaded to pure software emulation with later model revisions. I, too, would like to see info on this supposed "spare die space on the USB controller" claim. Because it would be really neat if true lol