It’s been thousands of years since the Greeks held Istanbul. The Byzantine Empire succeeded the Roman Empire, so it doesn’t count as Greek.
To its contemporaries at least before 1204, and maybe even afterwards, perhaps not. To objective historians sceptical of notions of ethnic continuity and romantic nationalism, perhaps not. To the people who claim Byzantium as part of their past, and who looked to the Patriarch of Constantinople as their leader throughout their time in the Rum Millet—of course it counts.
That, and everything I had to say about the subject in: Nick Nicholas’ answer to Do modern Greek people feel that Istanbul/Constantinople belongs to them?