I have no idea where it was implied in that comment that it was difficult. I said that it wasn't required to be considered a valid BIOS, as commercial/alternatives did the same.
I suspect he's referring to the "D Language Foundation[1]"
Mr. Bright, thank you for Empire. Some of my earliest tinkering memories are of upgrading my 5150 to have a V20 CPU and being somewhat pleased that map generation took somewhat less time.
https://en.m.wikipedia.org/wiki/BIOS_interrupt_call
Not ideal, but not a deal breaker. And it might go against Microsoft's IP (not sure, personally).