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.

Does this mean that the hibernated image must be signed each time the laptop hibernates?

The swap file that memory is dumped to during hibernation is on an encrypted disk. Upon wake, you need to unlock the disk before you can resume from hibernation.