I agree that an "ideal" SBMM mimics a coin flip. However, you are neglecting to include the fact that players get two Mercies. If you only have to win 7 of 9 games and you have a 50% chance of winning any given game, then your chance of going Flawless is around 10.8% (for a single card). That's not great, but 10.8% chance per card (on your first card) isn't bad.