As lordnacho says, that argument doesn't hold up at all. 20% seems really high, to me. A PhD in math isn't like scoring highly on an IQ test, it's not purely the result of being mathematically capable, it's also the result of undertaking a PhD in math. Plenty of smart people apply their abilities elsewhere. Perhaps in finance, for instance.
Also, I'd guess that the Olympiad winners who started a math PhD were more likely to succeed than the average math PhD student, and more likely than average to eventually get tenure.
> not being food at the competitions
I rather like this typo ;-P
I agree with the answer above. You need to pour a fair bit of effort into getting on an Olympiad team. Whether you get a PhD is more a question of economics, IMO. If you're showing that much promise there will be many other temptations, and bear in mind it's not just rich countries that have math kids, there's plenty of olympiad kids who need to get out and make money when they've done their degrees.
The kind of work you do in a PhD is also quite different. A math contest is a few questions with definite answers and a time limit.
It's slang for "unsubstantiated opinion likely to incite controversy". Weird how this word started appearing everywhere suddenly in the last few months.
In this case, by "that doesn't seem like a hot take to me" I mean that as far as opinions go this seems pretty uncontroversial.