Greeks had unified culture. There was the same language, lifestyle, music, alphabet, commerce, art style, military style... They called people outside their culture "barbaros".
dialects with regional substrate and vulgar varieties; koine written greek is a rather late standard, perhaps not byzantine, but koine literally means "written", in contrast to spoken.
> lifestyle
like what? Sedentary, sure.
> music
music-ians, who were not the same everywhere
> alphabet
again, regional variants exist, writing style wasn't fixed
> commerce
commerce is competition, so how could it be unified? associated towards outsiders, perhaps.
> military style
I wouldn't know about that, but Sparta vs Athens thousand years earlier would be a common counterexample
It's debatable how hellenized the Greeks on the anatolian cost were, the Lydiand, moesians and cetera. "barbaros" relates to speech first and foremost, though I am not buying the etymology. Speakers of other mother languages would have reasonable trouble getting the pronounciation right, so a Scythian, Tracian, Dalmation or otherwise "Celtic" Slave would appear hard to comprehend.
If you are generalizing a millenium of history, you are all history barbarians.
Could we agree that Athens and Syracuse were more similar than Athens and Persepolis?
How about common myths and goods? Olympic games that paused wars? Delphi were in "center" of helenic world? And ancient debate if Macedonians are "true Greeks" or "barbarians"?