And then from inside your link, there seemed to be even more critical voices:

"Then again, the Stoics were famous for challenging common conceptions, and the founder of the school, Zeno of Citium, had declared slavery an evil in his Republic"

He acknowledged their existence but as far as I know, he, not anyone else, said that slaving people was "wrong" or shouldn't be done.

It's important to note that Ceneca and the famous stoics (Marcus Aurelius for example) were rich and benefitted greatly from the status quo.

So now I think it is a shame, that I barely ever heard of him, despite having heard from all the others great (but slavery endorsing) greek minds. And sure, slavery was very common everywhere at that time. One more reason maybe to celebrate early free thinkers?