??? This is the first I’ve even heard this “theory”, lmao. It sounds like bullshit to me to the point that I’m not sure it’s worth breaking this down at all.
But it is worth noting that when the Rhoynar fled Essos, it was to escape the Valyrians, who would have certainly enslaved them and subjugated them otherwise. As far as I can tell, slavery was not a construct that was practiced in Rhoynish society, so it would make no sense that they’d practice it upon coming to Dorne. And if they did, it would have surely been noted.
Fast forward to present day Dorne— there is still no evidence of slavery being practiced, in secret or otherwise. They are a society whose hallmarks include greater amounts of personal and societal freedoms. There has never been a hint of slavery being practiced.
For for Areo Hotah, he comes from Norvos, and his role in Mellario’s household had been that of a guardsman. He may have been a slave in Norvos, as he was sold to the bearded priests to conduct his training. While he may have been brought to Dorne while been considered a slave in Norvos, such bonds certainly ended upon arriving in Dorne. That is confirmed by the fact that he did not return with Mellario to Norvos, but instead remained in Dorne as Doran’s captain of the guard. A slave would have been obligated to return with the household they arrived with.
I can see how Areo’s service to House Martell may seem like a gray area if he was indeed a slave in Norvos. But to imply that there’s a whole secret web of slaves in Dorne is ridiculous.