You can't just say this, you have to say why. It's going to be a hard case to explain how you can't compare two types of work.
I'm not even sure how you can explain what free market labor is without referring to slavery, or explain what slavery is without referring to free market labor i.e. I don't think it's possible not to compare them and rationally talk about either of them.
Nope, 300 years ago we had slavery and now we have gig work. I think that's the same universe ;)
In all seriousness, refusing to compare two things because you think they're "just too different" is simply acting irrationally. Look at the objectives, pressures, and outputs of each system and you may see more similarities than you'd like.
I'm not even sure how you can explain what free market labor is without referring to slavery, or explain what slavery is without referring to free market labor i.e. I don't think it's possible not to compare them and rationally talk about either of them.