I think "turning on the MMU" is something a bit different than "turning on virtual memory." The former just means that all addresses are translated from virtua

I think "turning on the MMU" is something a bit different than "turning on virtual memory." The former just means that all addresses are translated from virtual->physical, while the latter implies more abstraction: virtual memory can be overcommitted, pages might not be resident. This is just a guess, but I'd wager that UEFI has the MMU enabled, but with an identity mapping that maps all virtual addresses to the same physical address.