Quotes are always open to interpretation. People then were writing contradictions as we do today on these forums. There were Slavs for sure. But no more than 8% overall. And if I am not mistaken. Nicephorus deported many Slavs to Anatolia, sothat they could be absorbed by the masses there. In the 9th century, the Slavs laid siege to Patras and lost. A few decades later we have documents of some Slavs revolting. Laying siege and revolting is indicative of a minority exogroup. The same is true for deportation. How can some people come and deport you against your will you have significant numbers. You only allow that to happen if you don't have significant power. And sometimes the Slavs fought with assistance. I.e. in the siege of Patras they were accompanied by Saraces. Why would they need Sarances to share the spoils if they were numerous enough?
It also caught my attention that the Greeks in the 10th century reffered to the Slavs as Scythians. So I don't think they were very Balkanic. As the Scythians are identified with the region of the Ukraine. Paleo-Balkan people however were pary of the Hellenic sphere by that time. Genetically they were quite similar to Greeks, and culturally they would have been Christians. These Slavic 'Scythian' newcomers were certainly not. Hence the term Scythian.
The Slavic numbers are vastly overrated. Mainland Greeks are related to other Balkaners mostly due to early migrations. Because Greeks colonized these areas, migrating tribes in ancient times and because the overlap from Hellenic/Aegean to Thracian/Illyrian was very gradual and overlapping.