Never heard this claim, but it's a weak argument to rest on if we believe it to be true.
Naturally blonde and red haired black people arent that uncommon. Furthermore, if we are being honest, every genetic trait on this planet is represented in a subset of the African population.
Every variation of hair color, eye color, skin color, eye lid, etc...naturally occur among the population.
Yes, they are, which is why every reference to “naturally blonde black people” focusses on one group—Melanesians of the Solomon Islands—as the emblrmatic group, of whom about 5-10% are naturally blonde, which would be an incredibly tiny share of the overall Black population, if they were racially Black in the first place, rather than dark skinned Pacific Islanders.
There are over a billion racially black (by recent sub-Saharan African descent) people in the world. “Literally millions” is tenths of a percent of that population—quite rare.