Yes, Pop is short for “popular”, but I think it’s a genre of its own, and doesn’t just function as an abbreviated adjective. And sure, there’s plenty of Pop genre crossing, like Pop Rock, Pop Punk, etc., but plenty of genres can cross, like Country Rock. Some music arguably fits only in the genre of pure Pop, like what Taylor Swift is doing now (as opposed to the Pop County she used to do). So I think the term “Pop” is more than just an adjectival descriptor.
You could technically consider, say, “Shake it off” a Dance tune. Which is it’s own genre or, more specifically, bunch of genres.
I don’t think the “pure pop” genre really exists by definition. Pop feeds off other musical genres, simplifies them and makes it all about the hook, but it doesn’t exist on its own.
When I describe music as pop it’s generally because they have a certain sound, not because they’re necessarily popular. It’s more or less just a genre. Just like indie music isn’t necessarily independent and Alternative music isn’t necessarily alternative, pop music isn’t necessarily popular.
And yeah, to answer to question, as others have said sometimes rock is a bit pop and other times it’s not.