In this case, unlockable means it can be unlocked. As a native American English speaker, I have not seen unlockable used as "cannot be locked", but both definitions are in the Oxford dictionary. If I understand correctly, a locked bootloader prevents you from installing a different OS or rooting a device you might "own".

"Unlockable" == "The option exists for the user to unlock the bootloader if they wish to do so."

Even as a native speaker, it can get awkward explaining 'unlockable bootloaders' because the impliction is that they're first locked if they can be later unlocked... when there are also locked bootloaders that CANNOT be unlocked.

It's better to just assume 'unlockable' means 'not restricted.'