I think there are several different meanings of pop music. Yes, it can mean anything that has mainstream appeal, but it can also be a genre in of itself. It can mean anything that is melodic, not too hard or heavy, more tending to rely on live instruments rather than purely electronics. You can record pop music without being famous or that successful.
But in this instance, the record label may mean it as just non-classical. Tori's album is being released on Mercury Classics and her last album was classically inspired so some people might think this is another classical album. So I think they just want to clarify its her return to her previous less totally classical style of music.