>Calling them "Greeks" is about as accurate as calling the old Vikings "Norwegians" or "Swedish". Related by descent, but not nearly the same, and almost 1000 years separating them.

I mean... wouldn't the people of the British Isles during the Viking Age have called the raiders Danes, or Norsemen? Not vikings?

If your invaders speak Greek and come from the area of Greece, calling them Greek seems to make sense, albeit obviously not in a way equivalent to Hellenes.