My brother says it's impossible to hit higher than plat on his account due to his "hidden mmr" holding him back

I don't play six siege personally the game is too slow for me so I stick to playing other fps games but my brother has playing siege religiously for years with a team and he never even hit higher than plat once.

Every time I make fun of him and say lol skill issue, he starts blaming external factors "oh it's because you need a team in this game" and the latest one "it's my hidden mmr holding me back". "You don't get it I am playing against champion players in silver and gold and my hidden mmr makes it so I can never climb too high before the season ends and it resets all over again"

And I am thinking what a load of bs, because he's always saying he's goated in the game but brother you are gold 4 what delulu world are you living in?

Please tell me if he actually can't hit higher than plat and feel free to roast me if I am wrong in my assumptions.

P.S I went from bronze on dps in overwatch all the way to plat all solo queue and I could always see that it was my fault if we lost a game but my brother always blames external factors and it pisses me off