I once heard from a Frum historian that Yochanan Hyrkanos conquered and then enslaved the Idumeans and made them all Avodim C’Naanim. However, they allowed them to maintain some self rule. Hordus’ father Antipater was the ruler of the Idumeans, but still technically a halachic Eved.
Therefore Hordus was an Eved in Halacha but was viewed by the Romans as being of Noble/Ruling stock. As such when he took over he was able to get Roman support, but when all is said and done, Halachically he was an Eved.