reason that sticking to the documented library and systems calls was requested was mostly to reduce testing effort and to ensure that the game remains compatible with future hardware, features or add-ons
That's not to say people stick to the rules especially when on the PS1 there were more efficient ways of accessing the hardware directly.
Chucking in the last version of a third party library is cool until you inadvertently introduce a security issue or melt the processor.
I sort of understand that many people see Apple in terms of the 'iPhone'.. rather than desktop
theres nothing stopping you from using open source on the Mac and shipping a product, with a few exceptions.
But honestly Apple M hardware is great for development, solid Unix O/S , comes with many of development tools for free , fast and power efficient, hardware acceleration .. and
silent.