I think the confusion here stems from the fact that with historical slavery like the transatlantic slave trade, both slaves and slave owners are long dead. So the only parts of that slavery that still affect living people are the longer-lasting cultural effects, such as racial inequality.
Personally, I think we should probably be addressing those cultural effects, and that seems to generally be what these “reparations” are about? But I do agree that modern slavery is something that really needs to be addressed as well, and living slaves absolutely deserve reparations from their slavers.