It also used to encompass gender, up until Gen III where people just assumed Legendaries could only have one gender, which once again, was broken. The "can't breed" part had to be adapted to "Can't breed to make more of itself" since Manaphy came along.It has a 600 BST, cannot breed, and isn't part of an evolution line. That's pretty much the exact definition of a legendary otherwise all the other 600 BST aren't Legendary either.
Then there's Phione who can breed to make more of itself and is pretty weak, yet is a legendary by proxy.
If I recall, "Legendary" used to just be a fan term, we really shouldn't pretend its definition is set in stone when we gave it that definition in the first place. Perhaps I'm thinking of Pseudo-legendary, but its not like Game Freak haven't gone and retconed themselves in other places.
Too bad the Ranger Sign wasn't explained that well. I'd still like to know the mechanics of it.
Also, the animation seemed bad to me compared to other episodes. Why can't they get the good animators for these types of specials >: