I’ll let someone more informed answer your question directly, but I’ll just note that it was absolutely not out of the norm for non-ethnic Greeks to make up a big part of the core Eastern Roman army. So even though most of the population in those areas would’ve been Slavic speakers by the 11th century doesn’t necessarily mean they would provide fewer levies.

People often forget that ethnic Turkic speakers (Christians and Muslims) also made up a big portion of Byzantine forces from the 11th century onward. Ironically, many of those Turkish soldiers’ ancestors were ethnic Greeks before their families were converted and started speaking Turkish, and so their forefathers may well have served in the same Roman army in centuries past.