The very same ancient Greeks we are both referring to felt that the exact same system as applied by the Celtic speaking folks in Bulgaria to the German speaking folks there was, in fact, "slavery".
Look, admit it, the Greeks were simply trying to whitewash this business of holding fellow Greeks as slaves. It's time to put an end to their hypocrisy and call it what was.
Not at all. It never occurred to them to "whitewash" slavery--they took slavery for granted, to include holding fellow Greeks as personally owned slaves working in their household or farms.
If you want to adopt an expansive definition of slavery and categorize the status of the Greeks in the conquered towns & villages of the Messenians as slaves--fine. But they were in a very different category of slavery from the personally owned slaves of the Greeks.