Equating North Africans who genetically, culturally more interlinked with other Mediterranean people with Sub-Saharan Africans is not exactly right. After all before the Indo-European invasion Europe was almost exclusively settled by middle eastern immigrants (and largely remained so in Southern Europe genetically).

> It's not a stretch to imagine

It’s not a stretch to imagine a lot of things but there is no evidence that was true to any significant degree (besides the demand for slaves in Roman Egypt was likely substantial enough to absorb most of the demand)