With the caveat that ARM isn't a industry standard like PC has become, thus while propritary OSes can thrive, FOSS has a much higher challenge other than OEM specific distros or downstream forks.
Practically speaking, very few systems actually support SystemReady. There's an experimental port of edk2 for the Raspberry Pi, but some hardware is unavailable when using it.
It's weird that the book is titled "PC98", because PC-98 usually refers to NEC's line of x86 but not IBM PC compatible computers: https://en.wikipedia.org/wiki/PC-98
Which I think reinforces your argument: there were non-standard x86 platforms, but thankfully they died out. Given the situation of the home computer industry during the 8 and 16-bit eras, it is a small miracle that we ended up having an open industry standard.
Stuff like this, https://www.amazon.de/-/en/Microsoft-Corporation/dp/15723171...