There tends to be a particular "sound" to hymns, so it makes great sense to talk of hymns in the sense of musical genre, apart from any theistic orientation. Sure, there's musical cross-over with non-theistic songs (from which hymn writers lifted heavily), but in contrast to hymns, the secular counterparts tend to be story-type songs of murder, betrayal, licentiousness, courtship, coercion...--not quite the same thing. I think the request for "atheist hymns" refers to uplifting, inspiring or gospelesque songs with a similar sound but without the theistic baggage--that great leap from "isn't this amazing?" or "let's create a better world" to "we must be the undeserving pawns of a loving but strangely violent deity who demonstrates his love by testing our faith in ways that would give the Marquis de Sade a wet dream, according to some perfect and perfectly inscrutable plan--and if you can't accept this, may you burn in torment forever while we sit around with harps, eat bonbons and remind god what a ripping chap he is."