How could your statement possibly be true? This is hugely correlative.

"Good at one thing" in children generally implies "parents are involved" which generally implies "better than average socioeconomic status" which generally implies "children at least relatively decent at most educational things". This is going to be doubly true for things like art or music (or anything with private coaching) which require a significant monetary outlay in order to be good.

About the only place where this doesn't hold are the extremes where high spectrum people can function spectacularly in math and science and yet be ferociously terrible outside of that.