Also -- Terence Tao has pretty broad interests already within mathematics.

And I'm starting to suspect that the category "mathematics" is like the category "single-celled organisms." It's a convenient notation to lump together a class of objects that most humans ignore, but that are intrinsically much more variable than all the other objects that are more salient to most humans' lives. Two single-celled organisms can be more different from each other than you are from an oak tree. Commutative algebra and non-commutative algebra can be more different, separated by a wider gulf of background knowledge, than genetics and economics. (That's a little hyperbolic but I think some statements of that kind are actually true.)

So I think people with broad mathematical interests are really more "polymathic" than we tend to realize from the outside. Math is diverse. Terence Tao may in fact be more of a polymath than Tyler Cowen, if we're talking about expertise in radically varied fields that are rarely studied together by one person.