Classical period

By the 5th century BC, the Thracian presence was pervasive enough to have made Herodotus (book 5) call them the second-most numerous people in the part of the world known by him (after the Indians), and potentially the most powerful, if not for their disunity. The Thracians in classical times were broken up into a large number of groups and tribes, though a number of powerful Thracian states were organized, such as the Odrysian kingdom of Thrace and the Dacian kingdom of Burebista. A type of soldier of this period called the Peltast probably originated in Thrace.
A Thracian coin from Panticapaeum, bearing the inscription ΠΑΝ(ΤΙΚΑΠΑΙΟΝ) and Macedonian symbol of the Vergina Sun inside a diadem. 2nd century BCE.
A Thracian coin from Panticapaeum, bearing the inscription ΠΑΝ(ΤΙΚΑΠΑΙΟΝ) and Macedonian symbol of the Vergina Sun inside a diadem. 2nd century BCE.

In that period contacts between the Thracians and Classical Greece intensified which led to strengthening Greek influences in Thracian society, culture and handcrafts. Because their language had no written tradition, in some regions the Thracian aristocracy and administration adopted Classical Greek for an official language and Thracian merchants utilised it as a 'lingua franca' in their contacts with other tribes and peoples. As a result a level of Hellenization was observed in the following centuries and it was deeper imposed by the Macedonian conquests over the Thracian territory in 3rd century BC.

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here we have evidence that greek was used as a lingua franca, through necessity. if the 'hellenization' of the thracians was intensified by the Macedonian conquest of thrace - as the wikipedia article on thracians says - then one could argue that the Macedonians had also earlier adopted greek as an (administrative) lingua franca. the use of a written language by the burgeoning Macedonian empire was as important to them as it would be to any administration today. A written language was necessary to issue laws and decrees, make official announcements, and enable education, business, trade, military plans, etc. Remember too that greek was one of the very few written languages in the whole world at the time, and a major mediterranean (middle of the earth) trade language, so it made sense as an Aegean lingua franca.

There is no doubt that the Macedonians used the Koine for lingua franca. It's just up the fellow Greek buddies that doesn't accept it. Btw Barbarian, i think you mixed up some stuff a bit. The Ancient one, is called Hellenic Koine. The modern one - Just Greek, it's a type of Bastardized Koine language.