I would think they have been getting royalties on episode reruns, since they were tracked down via a cue sheet, which has their names on it. Cue sheet is used by performing rights organizations to pay out royalties to performers. Given the story of the track's composition ("we need a song, you have four hours") my guess is that these folks have produced a large number of songs over the past 30 years that earn relatively small royalties per track. Add on top of that there are multiple tracks called "X-Files" just in this episode and I'd further guess there are plenty of songs that the musicians are getting paid for that they have forgotten over the years!