He has at best an intelligence a bit above average, tough seems to believe he's some kind of genious.

Like in so many things, there seems to be a sort of Dunning-Krugger Effect when it comes to intelligence, with the "just above average" up to what I call "entry level genious" - i.e. those just about intelligent enough to frequently outsmart most people - often vastly overestimating their own mental capabilities.

(Think Elon Musk if you need another example).

It probably doesn't help that in the environment he grew up and lives, those who intellectually can run circles around him won't actually tell it to his face and instead just use him if they can or keep quiet to avoid some kind of negative reprecursions if they're in a position of less power.

What he does have which is vastly superior to most people is the lack of scruples and of morals: there are definite advantages in terms of outwittting others for those who lie and deceive with the same naturality as breathing. That is however not intelligence, just a mix of sociopathy and public school training.