What Britannica says about the early populations of the area 

Macedonia: History – Encyclopaedia Britannica

Macedonia owes its name to the ancient kingdom of Macedonia (or Macedon). Centred in the southern part of the region, (i.e. present-day Northern Greece) this kingdom seems to have been largely Greek-speaking, with Thracian and Illyrian admixtures. By the 4th century bc, it had extended its rule northward into the Balkan Peninsula and throughout the Mediterranean. In the 2nd century bc, Macedonia was made into a Roman province. When the Roman Empire was divided in the 4th century ad into eastern and western halves, Macedonia became part of the eastern half, which became the Byzantine Empire. By this time the population of Macedonia had been largely Christianized. Macedonia’s Greek ethnic composition was overturned by the invasion of Slavic peoples into the Balkans in the 6th and 7th centuries AD.

Paionia (historical region) – Encyclopaedia Britannica

Paeonia,  the land of the Paeonians, originally including the whole Axius (Vardar) River valley and the surrounding areas, in what is now northern Greece, (Republic of) Macedonia, and western Bulgaria. The Paeonians, who were probably of mixed Thraco-Illyrian origin, were weakened by the Persian invasion (490 bc), and those tribes living along the Strymon River (in western Bulgaria) fell under Thracian control. The growth of Macedonia forced the remaining Paeonians northward, and in 358 bc they were defeated by Philip II of Macedonia. The native dynasty, however, continued to be highly respected: about 289 bc, King Audoleon received Athenian citizenship, and his daughter married Pyrrhus, king of Epirus. Under the Romans, Paeonia was included in the second and third districts of the province of Macedonia. By ad 400, however, the Paeonians had lost their identity, and Paeonia was merely a geographic term

It is incorrect to citing Britannica in this article biased, i.e. only the article about the history of the are, which is today Northern Greece, ignoring the article, which points to the history of today R. of Macedonia (Paionia).