These sources just highlight that he was, at best, under threat of being a slave though, not that he was actually sold and performed as a slave. He's ransomed for at the last minute by someone recognizing him at the market.

If Plato had actually been a slave, his lifelong commitment to anti-democratic political organization would've been a little different, though maybe not, considering even this experience didn't change his philosophy, to the point that's likely not worth mentioning as a significant experience, unlike Epictectus who was born into slavery.