If anything, David Olusoga understates his case. There is no such thing as IQ. I have never taken an IQ test in my life. I question whether anyone who sets any store by them is sufficiently intelligent to be allowed out alone, if at all.
The whole thing depends on “mental age”, whatever that may be. The IQ of children in numerous countries has “improved” dramatically over the years that IQ tests have been set, and therefore taught to, in schools. Indeed, that never fails to happen.
The publications of Mensa are a particularly rich seam of amusement. “More people than you might think are above average”? I am guessing that it is about half of them. “One person in 20 is in the top five per cent”? You don’t say! And so on. But never try and tell the “I have a high IQ” lot any of this.
You wouldn’t have to, and indeed you never could, do anything to get a high IQ, even if such a thing really existed. Having it would be no cause for congratulation, still less for self-congratulation or for the creation of an international society for mutual congratulation.