I was thinking about that too but I'm guessing he let it slide bc he could still play through a dustshoot. He got the opponent to use two high value traps and only resolved one of them which didn't really hurt. In case BW player had mst as an out baiting out mirror to negate it with decree permanently removes mirror force from the deck for the rest of the game.
@Neros_Palace thats a good point. If i had EVERYTHING i need and didnt care id possibly hold the decree cuz i dont wanna get hit by icarus or solemn. I doubt thats the reason, but VERY POSSIBLE.