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.