Sounds like a misnomer to me.
Well, something has to be. You can have your EFI partition on a separate drive and then the actual drive will be fully encrypted. It’s just as good as we can get, the algorithm for decrypting the data obviously can’t be encrypted.
I think there are implementations with encryption logic stored in the BIOS or on a separate chip, but don’t quote me on that. And even then, the decryption logic itself will be unencrypted, because, as it happens, computers can’t run encrypted code.
Why not have the BIOS decrypt the disk then continue the boot process as normal?
Mainly because then the manufacturer decides on how your stuff is encrypted, no likie.
Must be because full-ish sounds way too much like foolish, making people think it is a useless thing to do.