It helps that unlike with BIOS, the modern tooling doesn't have huge amounts of "we never read the spec but it booted on my machine" that was prevalent even wit

It helps that unlike with BIOS, the modern tooling doesn't have huge amounts of "we never read the spec but it booted on my machine" that was prevalent even with grub2. TL;DR a BIOS doesn't have to boot a drive if there's no primary partition marked "active", no matter whether your boot code uses that or not.