There is no any “UEFI boot protocol”. So this non-existent “UEFI boot” or how they (uboot) call it, might be anything, but UEFI support.
UEFI shell by the way isn’t part of UEFI, it’s just an UEFI application implementing some protocol, the one that is not part of UEFI.
Actually it’s all that simple. There is a “requirement” part in the specification. It lists everything needed to have been implemented by a product for it to be UEFI-compliant. Apart from compliance testing of course. Until then uboot isn’t even closely UEFI. it just has some dirty hacks for pretending it is and being able to do 0.01% of what UEFI compliant FW should be able to do.
Better for everyone wishing to have UEFI, and not seeing capabilty to create it from scratch, to stick up with edk (Tianocore). And forget about uboot.