It means that the Beatles (who the parent mentioned explicitly as a basic formulaic rock in contrast to the "peak" from the '90s) is much more well known now than Radiohead, and could even have been more popular than Radiohead even during the peak of Radiohead's popularity (with the general audience, of course). I doubt that the problem of alternative bands was media exposure, because they had plenty in their heyday. The issue is that their music appeal only to a small(ish) subset of people, and that most people would like something else. I don't mean this as a bad thing. It is what it is, but I find amusing how cultish their following is :)