> this didn't happen centuries ago, this happened in the 1960s to 1990s
The point is every ethnic group was made up by people at some point. Most retroactively. There isn't a rational reason to prefer old over new ethnic identities.
Culture is what you do when you stop thinking. When everyone in a group does the same thing when they stop thinking you have a nation.
Give it another 60 years and Macedonians will be on par with Israelis for a made up nation that is now real. We aren't there yet because there are people alive and well in their 40s who know just how made up it all is, since they were the ones that made it up.
This is a very hard concept to explain to liberal americans because they have no culture or nation. Thinking the wrong thoughts gets you excommunicated and what those wrong thoughts are changes constantly. So they have to constantly think about what acceptable behaviour is.
The Macedonian culture seems to me to have become distinct from Serbian and Bulgarian cultures by around WW1 at the latest. At the very least, all of the ethnographers being used to divvy up the Balkans recognized them as a distinct ethnicity at that time.
The point is every ethnic group was made up by people at some point. Most retroactively. There isn't a rational reason to prefer old over new ethnic identities.