Nobody's favourite gamemode, sure. But you need 5 SQDM wins to complete one of the assignments I happened across a server that has the score limit set to 2 - so lots of rounds go by and you get to 5 without spending too long away from the real game.

So it's a legal way to cheat?

I don't know about cheating. But it's not a very popular mode and it's a real PITA to try & win 5 games with strangers. Extremely short rounds is still playing the game, and you still have to be on the winning team 5 times. Default settings might result in half-hour rounds, this is more like half a minute and therefore the 20 rounds (on average) you have to play to get the unlock are over that much more quickly.