That's what they're calling it, but it doesn't sound alternative to me. Maybe it's just because a lot of those songs, especially from the mid-90's, eventually became hits and are now familiar. If I were to rewind back to college, this iteration of 99X would sound a lot more like top-40 than alternative. A lot of songs the alternative station in my college town played aren't being touched by 99X, though one could likely make the case that those songs have long since burned with most people my age. Cumulus doesn't do much of anything haphazardly. So, I'm sure it was well-researched, and the songs that are no longer widely popular have been removed.
They can call it what they want, but, to me, it fits a lot better alongside classic hits and AAA than it does any of today's alternative or classic rock stations.