Hopefully it is not "free for all" but it has some internal not visible ranking or at least classification, otherwise even if they change the fact that there are mirror matches, people still won't play it because they will be steam rolled. Doesn't mean that there should be a ladder of some sorts, there could simply be classes... if you win X amount of matches in a class you proceed into next one, and you get matched in the same class if there are enough people searching.
How does it really work? Well, only FLX or someone like that can tell you, which is highly unlikely.