The back end is swappable. I think right now you can use Mongo or MySQL. If you want to use something else you have to write your own DDL. They hope that more people will do that in the future. Or presumably, they will do it themselves, once they think it's a priority.
EDIT: To be clear, I'm not necessarily knocking MongoDB—people who know more about it than I would do a better job of evaluating that. But realistically a swappable DB backend isn't that difficult, and if it is, they should seriously reconsider how they are using the database.