It doesn't, it's just another bootstrapping method that happens to work fine with hibernation. UKI allows you to extend your chain of trust from the bootloader

It doesn't, it's just another bootstrapping method that happens to work fine with hibernation. UKI allows you to extend your chain of trust from the bootloader to ramdisk, instead of just your bootloader and kernel. From there, you can enable kernel lockdown and checking of module signatures if you want to. I think you can do the same thing without UKI (I forget tbh), but UKI simplifies it with one UEFI executable that doesn't even need a bootloader.