"similar skin colors can result from convergent adaptation rather than from genetic relatedness"

Well... duh. I don't think anyone would have expected that the reason sub-Saharan Africans, south Indians, and Australian Aborigines are all dark-skinned, or Europeans, Ainu and Inuit are all pale-skinned, is that they're closely related.

Classifiers such as "black", "white", and so on do not carve nature at its joints.

Those labels aren't intended to be literal. Colin Powell is still generally considered “black”, despite being pale-ish.