UN estimates 300,000-400,000 enslaved in Eritrea

This is kind of a yes and no thing. The Arab slave trade was far, far worse than the Trans Atlantic one. Of all the peoples involved, Europeans are the least culpable. The Africans were the slavers, the Arabs bought far more and kept the practice going for far longer. The Africans had to be forced by the British empire to cease the trade. The hundreds of thousands of European slaves in North Africa and the Middle east are never mentioned, nor are the slaving raids on the European cost by the Barbary States.

However, America tends to focus on America, so in The States you have the descendants of slaves and the descendants of slave owners, the Trans Atlantic trade is always going to feature more heavily and thus the perspective of slavery will always be skewed.