“Players who have a high CHA AND come up with decent ideas, roleplay well, and make a good argument will obviously do better than people with a high CHA who always think up dumb ideas and/or don’t roleplay their character and/or can’t string together two words. But having a high CHA still means statistically, you’ll do better than you otherwise would on account of your PC being likable even if you aren’t capable of playing him as such.”

This is completely incoherent. He’s saying on the one hand you must role-play your interactions, but at the same time your interactions will be helped by your (probably randomly rolled) high stats. If he believed his own argument, he would be forcing his socially-awkward players to drop their charatcer’s CHA by ten points. Or he would be dropping the CHA stat (and INT, for that matter) from his games entirely and demanding pure roleplaying.