Yes, Interrail has nothing to do with EU so all Europeans, and even people from Turkey, can use Interrail.

North-Macedonia is IN he system-and this means that even the home-country rule is valid: can use the very, very few trains-if you really wanted-in there only on 2 days. And as you likely know: there are NO voz/vlak over its borders for now-and also not further on from Serbije.

The only exception in this all is Albania-had trains but was always out of accept passes-these people can buy INterRail too. As can Byelorusski, Russky, Ukrainsky, Moldovians.