So I guess a Boss 302 or 302 Z28 isn't a muscle car. Or maybe a 375hp 'Vette?
But I agree, muscle cars were pretty much "60's and 70's" Who knows who first 'coined' the word, probably some magazine writer. I don't think "back then" anyone ever considered any wagon or more door a muscle car. It pretty much had to be in the "sport" class to be considered. And, "FACTORY performance car" was what the original name referred to. You could "build your own" car, but that is not what the name originally referred to.