Can this UEFI firmware be ported to other ARM devices, e.g. phones, tablets, books?

Yes, it's just a port of the EDK2 UEFI reference firmware

Have you checked out uboot?

https://u-boot.org/

Doesn't everything under the sun boot with uboot? Uboot is usually what people want to replace when they say "why can't this just run UEFI?"

I have yet to find a valid reason for UEFI to replace u-boot, or UEFI to exist at all.