This doesn't make sense to me. Everyone can distinguish relative pitch, that's how humans understand music. It was probably a crucial part of human evolution, somehow, as we all can do it. Tonal languages use relative pitch, not absolute pitch. Everyone can learn Cantonese regardless whether they have AP or not. If you claim tonal languages train you for AP that's citation needed, I don't think that's true (we would expect tonal language speakers to be more likely to have AP, a trend we haven't observed).