Byzantine slavery was a direct outgrowth of Roman slavery, and Rome was undoubtedly a slave society, with anwhere between 4.6-19.3% of the population enslaved (see Kyle Harper, Slavery in the Late Roman World, 2011). Specifically on Byzantium, a huge, huge part of Byzantium’s Black Sea trade was in slaves, and Byzantium not merely acquired slaves by the tens and not hundreds of thousands at a time for use in a variety of industries at home, but also sold slaves widely across the Mediterranean, there were slave markets in many Byzantine towns and cities (the one in Constantinople was known as the “Valley of Lamentations”). So I think it counts.

For more on this, check out Youval Rotman’s Byzantine Slavery and the Mediterranean World (2009), and Hadjinicolaou-Marava’s’s Rercherches Sur La Vie Des Esclaves Dans Le Monde Byzantin (1950), which are both important monographs in their own right but should also provide some useful bibliography.