I assume you get why it's not a nice thing to say that "slavery helped black people in some ways" but I think some people miss why this particular idea is *exceptionally* racist.
One of the main arguments to justify slavery was that it "civilized" black people by making us Christian & teaching us obedience to our "betters." Black people were "lucky" or "better off" as slaves.
It assumes that when people were enslaved they had no skills, nothing to offer, that they were basically animals.