Not every person plays cod. I guarantee if you loaded up siege on console you would see why mnk is a bad idea. Siege has aim assis and you can turn it off. Personally I have it off. But for new players and people who aren’t in the conventional sense “normal” they need a little help playing. And being competitive is different to abusing actual cheating and dropping 15k 0d in a ranked game against controller players.
His enjoyment is for him as important as yours for you. He is in fact entitled to play only games he likes to play is setup he finds fair.
And he is fully entitled to express it out loud so that similarly minded people can bond or argue for existence of games they like.