I don't know about that: it might be an ancient university superstition. True whiteness comes from the W gene in horses, and I think can be found in many breeds. Some breeds are always grey, like the Camargue, but not white: but then some people think that white horses may just be ultra expressions of the grey colouring. Argentina is most famous for the Appalloosa (sp?) which is spotty.
Then again it might be true and i'll have to start my research all over again! If you find out let me know....