Why is it that people automatically want to carve up our history and redefine it.
It actually tends to be the other way around.
The Ancient Macedonians were most likely Illyrian in origin, but the ruling class eventually adopted Greek cultural characteristics. Even wikipedia states the possibility (not that wikipedia in itself is a credible source, but it shows that the belief is in existence.):
Besides the theory which regards Macedonians as a Greek-speaking tribe (Masson, Hammond), the Macedonians were sometimes spoken of as a tribe of Thrace, the land north-east of Greece, akin to the Thracians.(Sir William M. Ramsey). Rather than a Greek origin, some argue that the ancient Macedonians had an Illyrian or Thracian origin. It is also possible that the ancient Macedonians underwent ethnogenesis syncretizing Greek as well as Illyrian, and Thracian elements (cf. Borza, et al.).
There was a movie that showed a Bosnian wearing a t-shirt featuring an American rapper. That Bosnian liked American rap music. If he began to indulge in American culture, and even spread it, eating hotdogs and hamburgers and speaking English and passing American ideals to all his Bosnian friends, does that make him American? Absolutely not. Similarly, Alexander was not ethnically Greek, but he felt that the Greek culture was equated with progress and intelligence, and was superior to others, thus he spread it across the world.
Alexander didn't unite the Greeks, he
conquered them! But he didn't incorporate the Illyrian lands into his empire, although he was perfectly capable of adding them too. Why do you think that is?
Sir William Woodthorpe Tarn, of the British Academy, regarded worldwide as having written the definitive work on Alexander the Great, states in the opening paragraph of his book
Alexander the Great that "Alexander certainly had from his father (Philip II) and probably from his mother (Olymbia) Illyrian blood!" [P 1, ALEXANDER THE GREAT, W.W. Tarn, Beacon Press, Boston, 1956 ]
His mother, Olympia, was a princess of Epirus, a southern Illyrian, or Epirote, region that was characterized by the ancient Greeks as barbaric and non-Greek. But today they try to claim this territory. So then, tell me, were the ancient Greeks wrong in their knowledge that the southern Illyrian inhabitants of this territory were not Greek? And only the present-day Greeks that are highly nationalistic at claiming Alexander the Great are correct, and know more than the ancient historians that interacted with those non-Greek people?
Alexander wasn't ethnically Slavic (they arrived after 6th century), nor was he ethnically Greek. He did spread Greek culture, but that doesn't change his ethnicity.