I cannot believe BIOS boot support is even close to the amount of code required for UEFI boot.
Before you say "UEFI is much older than 10 years old", please remember that UEFI only started being a default ever since Windows 8 (10 years old). Windows 7 can't even boot on UEFI (it requires BIOS emulation to be active). I have a system from 7 years ago that while it claims to have an UEFI BIOS, it is so full bugs it is unusable except to boot Windows. I mean, critical bugs, as in, you boot Linux with UEFI just once, you risk the UEFI var storage getting corrupted and the motherboard becoming a paperweight.