Two weeks after checking how games are played in the Fighter league, it appears the distinction is meaningless so the online leagues are closed for now.
The distinction was made so players could pick a game matching their play-style and have more chance of playing an interesting game. It wasn't expected to be very efficient, but two weeks of recorded games showed it is meaningless. It seems the vast majority of players pick a game blindly, often not even aware that the game is modded.
The distinction between Builders and Fighters is removed. Battleplan players still have to pre-make a game to find a suitable play partner.