The parent comment is saying "used to be", and that was actually very true before the IBM PS/2 and the AT clones --- IBM published the schematics and BIOS source code for the PC, XT, and AT in the PC Technical Reference books.

https://archive.org/details/bitsavers_ibmpcat618ferenceMar86...

Even in the "AT clone" era (better known as "IBM PC Compatible"), the standards were relatively more open and there wasn't much in the way of DRM at all.