Could also have been a purchase for their licensing or metadata. It sounds like Primephonic has put a lot of work into all of that, and if Apple wants to get into classical then it'd make sense to throw a tiny-by-their-standards amount of money at a known-good metadata set and accompanying licenses for the music.

(Though I agree, it'd be kind of silly for them to not let the people they've acqui-hired keep on working on the "new" app in their domain.)