I may go see if I can find something explicit, but I distinctly remember that when it was first announced, it was meant to be a mode where it literally treated every player the same, regardless of age of account, hero pool, etc. The closest thing I can find to such a statement was from the original trailer.
Here it is implied that there is no MMR, as anyone can be matched with anyone else. “Everyone enters Heroes Brawl on equal footing.”