No. No. No. MLG does not prove skill. You could've host booted to get that rank, or boosted for all we know. And there is not proof you didn't get it legitimately.
He never asked if it proved true player skill. He stated that he thought it was the most informative tool at your disposal in an xbox live party, and it is. It's the hardest playlist to abuse to your advantage, wether that be teaming with people to carry you or abusing K/D ratios.