Hmm. Untrue, I beg to differ. I managed to do it by following the instructions here.
This answer on the Ask Ubuntu thread, the same one you said, and I quote
both of those links point toward creating a Bootable USB Image that can boot in either Legacy or UEFI mode. This is different from having the Installed OS able to be in UEFI / Legacy.
I managed to have a full installation of Ubuntu for both BIOS and UEFI.
here is how I did it:
In the .txt file here scroll down to where it reads:
- Previous system, that is unstable (can be destroyed by updates)
then follow the instructions. I did and now I have Zorin OS install that can be used in both BIOS machines and UEFI machines.