No slave ever learned anything beneficial by being a slave. Those skills became non beneficial simply by virtue of how they were forced on them. Even shit like farming cotton now leaves a bad taste in people's mouths because it is tainted by that trauma. And the machinery required to render hand labor for it redundant was invented around the Civil War anyway, rendering any such skills anachronistic and obsolete.
And even if we granted that they learned some skills, exactly how were they going to apply it for their personal benefit? Does DeSantis think that slaves could just walk up to their masters and say "Hey Guy Who Owns Me Like Property, I've been doing this farming thing for a while and have gotten good at it. Would you mind if I left and started my own farm?"
Seriously, does DeSantis really think that slaves were set free so they could do things for themselves? Nope. They were kept at property until the day they died. This "they learned skills that they could use" is wrong on every level.