Who cares? In most of the Eastern world, Greece is known as "Younan" from the ancient term "Ionia" which was a region of Ancient Greece located mostly (aside from a few islands) in modern day Anatolian "Turkey" rather than mainland European Greece, don't see anyone caring about that misnomer.
Not to mention a whole host of other countries are also misnamed so this isn't unique to Greece vs FYROM. Oh & I don't even want to get into genetics & how majority of Balkan Slavs are majority Slavicized Indigenous Balkan people (this is why Bulgarians & Poles clearly don't look alike) so Macedonian Slavs are probably descended in large part from ancient Greco-Macedonians, Illyrians, Dacians, Thracians etc; in essence they probably do also have ancestral claim as well if it were to get to that (as do modern Greeks obviously as well).