https://en.wikipedia.org/wiki/Macedonia_naming_dispute

TL;Dr they're both arguing over stolen glory from Alexander, being racist at each other, and Greece is kinda still mad at them for supporting the Nazi invasion on the path to "Greater Macedon"

Also

https://en.wikipedia.org/wiki/Macedonia_(ancient_kingdom)

Yeah you can kinda see why Greece has beef with them claiming the name.

I guess they didn't want to be known as Thracians since they only get remembered by super nerds and dudes who go "aren't they they ones that ran away at Thermopylae?"

Seems like the Federation of Macedonian Cultural Artistic Associations of Victoria is pulling a bit of a stunt here. If it was clearly communicated ahead of time in the rules that only national flags are permitted, then they knew what they were doing.

Sucks for the performers.