He was referring to the convoluted style of writing that the Byzantines preferred. They considered the classical Greek authors to be the gold standard and did their best to ape the style. Not only would they constantly reference snippets of Homer or Aeschylus to prove their erudition, but they tried to jam medieval Greek into the metrical and grammatical schemes of ancient Greek. The result was an appropriately ‘byzantine’ mess of often tortured prose. (presumably like that last sentence)
Voltaire was also engaging in a bit of dripping condescension toward the spoken word. Medieval Greek had changed in the thousand years since Polybius was writing and frankly it didn’t measure up to his standards- much the way King James English or Shakespeare sounds more majestic than our own vernacular.
Someone should have pointed out how barbaric his version of Latin sounded.