From my understanding, it's really not. An unlocked bootloader means that with physical access you can install software on the phone without wiping it first. With root you have full (controlled) access to the file system. You can actually have a locked, rooted phone, but it means that if you want to change something low-level like your ROM, you may need to wipe the phone to install it.
Well, yes, an unlocked/unlockable bootloader is not the same as a rooted phone, that's correct. But the former often leads to the latter: for example, the process of rooting a Nexus phone involves flashing a new recovery system, which takes advantage of the unlocked bootloader.
If you don't have an unlocked bootloader, you have to resort to exploits and other flaws in the system. Releasing a phone with an unlockable bootloader is opening a door to rooting and many "unauthorized" modifications.
That's what I mean they're the same in practical terms.
