UEFI Secure Boot requires SMM (because it secures access to the key vault). Non-x86 architectures use something else, for example they could use EL3 on ARM. Either way it's a "super-supervisor mode" of sorts.
Well, it does not have to be SMM that does it - it's just that SMM is what we have already available so unless you have certain rare hw it's what vendors will use :(
