Yes, considering apple have been in the music business longer than App Stores have even existed.

I don't think that can be right. On the surface you're suggesting that if a company worked in a particular area before another company then the first company is entitled to use anti-competitive practices, which clearly isn't true.

I think the main point the (grand?) parent is saying is that "Apple is in the phone business" isn't a reasonable description of Apple. I agree that Apple hosting itself on its own platform is an abuse of market position (especially for something as peripheral as music), but to claim that music/audio isn't a/hasn't been core part of Apple's business model is ridiculous.

Ah, gotcha. Yes, that does make more sense, thanks. But I still don't think it really addresses the second point, which is the crux of the argument as far as I can see.

Is Apple charging 30% on all Spotify transactions through their phone/tablet App Store giving them an economic advantage for their own Music store?

It’s irrelevant for the example whether Apple was already in that space.