It's a material collection/data disc, just like this is -
There's no way anyone could classify that Sora no Kiseki UMD as a soundtrack just because it has a selection of SnK albums to listen to - and even if you did that would mean games like Katamari Damacy and Suikoden I & II (both PSP versions) were also soundtracks because they contain full music players within them complete with track names and shuffle options.