It wasn't just x86, it was the fact that IBM released the entire documentation for the PC architecture up to the AT, including the BIOS source code (which was copyrighted, so competitors could not just copy it, but they could certainly study its function and produce a different but compatible implementation.)
For the PC, things started taking a turn for the worse with EFI and SecureBoot, but before that it was quite open.
Yes of course x86 implies the BIOS and the historically very significant reverse engineering of it by Compaq.
However EFI and Secure boot have not changed anything at all. UEFI is a standard and most vendors use the reference implementation from Intel and its all at least as good as BIOS in terms of being documented.
For the PC, things started taking a turn for the worse with EFI and SecureBoot, but before that it was quite open.