people think about mathematics in an extremely heterogeneous way

Sure, that's true.

reducing human brains to one number as some sort of "math hit points" is silly

And I agree. But consider the setup: we have a person who doesn't quite know what he wants to do and who has shown no signs of possessing any "supernatural" math abilities. Could he turn out to be another Groethendieck? Well, sure, it's possible, but we are talking about the base population rate here, the chances are, let's say, not very high.

Now, it so happens that most math professionals have high IQ. That's not a coincidence, of course -- if your brain is insufficiently weird to see math "directly", you have to rely on the same dimensions of performance (working memory, etc.) which are reflected in the IQ score.

Trying out a profession has costs, sometimes considerable. You can't try everything on the off chance that it might work out -- you want to focus on the areas where you expect to do well. And someone with an IQ of 130 has much, MUCH better chances of becoming a mathematician than someone with the IQ of 80.