In this particular case: conceding is usually a part of board games, even if it's not in the rules. I've been in many board games where we didn't wait for every last meeple to be officially removed form the board. Ergo, the dm declaring the fight over without rolling for each extra minion is conceding, ad conceding isn't out of line with the concept of combat-as-boardgame.I myself don't find any appreciable difference between "calling it" and fudging. In both cases the DM is just choosing to no longer play the board game as per the rules of said game. So one isn't any better or worse than the other to me.
Obviously it can be done badly or whatever, but it doesn't have the same sense of violation of social contract that changing a rolled result does.