Compaq created a new BIOS from a clean sheet of paper design, not a reverse engineered or ROM dump. The code of the BIOS on the PC and PC/XT were copyrighted.
I heard a tale - but can't find an online reference - for the first Compaq PC BIOS containing a bit that said 'Not (C) IBM' in order to pass some software checks searching for the strings '(C) IBM' being present at certain specific BIOS addresses.
Compaq wrote a specification based on IBM's published code listing. Then they handed it over to a separate team to reimplement.
userbinator's post clearly mentions cloning the hardware being problematic legality-wise and I was replying to that. Nobody's saying the cloned BIOS is problematic legal-wise.