That argument seems to hinge on the idea that Africa is one country, rather than a continent of many. If the sultans of Zanzibar enslaved people from the interior of the continent, or those of one tribe enslaved those of another, it is not a deficiency of the enslaved people's self respect. Nor does it suggest that the enslavers didn't value their own history and artifacts. There were nasty people and monsters of all sorts in all sorts of places, but they enslaved each other, not themselves.
To suggest that this justifies the European colonists' looting of African artifacts makes no more sense than saying Sweden could loot the British Museum because the British were at war with the French.
And though it may be the case that once Britain outlawed the slave trade they did so aggressively, it was a trade they had previously enjoyed, and for which they were partially responsible. The seller is guilty of many sins, but the buyer bears some too.
And anyway, this should not devolve into a tu quoque argument. If you treat people badly and steal their stuff, it's wrong in its own right, even if the people in question are no better.