Apple giving you more than consoles do is damning with faint praise, the Mac bootloader is open but without any public hardware documentation it's incredibly difficult to do anything useful with that openness.
Apple can revoke it at any time. If a future update disabled or changed iBoot, there is no guarantee Linux would ever run again (unlike UEFI Macs).
Valve is not like Apple, they treat UEFI as a default.
Worth noting that this is a dig against the other consoles which do not allow this, not Apple who (in part) does.