It's all good. I'm a bit of a pendant for word definitions people are using. I think loose definitions lead to all sorts of subtle miscommunications. Most of the time it's not a big deal, but I do have a soft spot for Ronald Dahl, so would like to verify when claims.
For all of the articles I can find written about Dahl's oompa-loompa slaves, not one of them puts together an argument as to why they are slaves. They just make the claim, and surround it with statements of the opmpa-loompas being originally black native inhabitants of Africa, and they worked for a white-man Wonka. I can see how people's minds go to slavery when describing a unbalanced power dynamic between an exclusively black workforce and a white boss man, but innuendo does not make for a compelling argument IMHO.
If the book doesn't mention them being imprisoned, doesn't mention being forced to work, talks about them being paid a wage, and given the choice on how to be paid, then they are not slaves. We can make lots of assumptions about what is happening behind the scenes of the factory, but we could do that just as easily to suggest the Oompa-loompas are actually the masterminds behind everything, and secretly control the governments of the world.
Personally, I think Dahl's original depiction was racist, but not describing slavery. The fact all these articles don't provide anything from the original text* showing that they are slaves makes me think this is a case of people wanting to inflate someones misdeads in order to generate clicks. It's easy enough to find real things to criticize the man for, but they should stick to the facts, or make an actual argument.
*The biographer was doing the same thing as the articles.