Many historians today from Yale, Oxford and so on consider the Ancient Macedonians as an Ancient Greek Kingdom ((which i'm not trying to debunk)).
However Demosthenes refers to Philip/Macedonians as barbarians, which was before Philips conquest if I remember correctly.
Also was the term Barbarian not used to describe those who where non Greek. Was this mere political rhetoric to further his agenda? Ancient Macedonians did speak a different dialect known as Doric Greek which too my knowledge was unrecognizable to Southern Greeks so could this have been part of his reasoning?
Whether or not the Macedonians were Greek was an issue on which the Greeks could never agree, because the decision was deeply political. There is no doubt that Demosthenes described Macedonians as the worst of all barbarians because it was his mission to unite the Athenians - and ideally all Greeks - against the growing power of Philip II. I wrote about this in more detail here.