I think this statement isn’t true. There isn’t anything right now remotely looking like an exploitable widely adopted interoperated music streaming ecosystem either built on AP or built on anything at all.
It’s important to realize we’re not trying to build a new music listening paradigm off of existing social networks. On the mainstream side, people don’t go to Twitter or Instagram or whatever to listen to their music (or movies, or videogames). They might discover that some music exists through those (much like they can discover that movies and videogames exist through those) but these tools are fundamentally not built to manage the complex states of music metadata and listener relationships.
Like, I feel so far, the AP integration of any music related services is pretty limited and not particularly speaking for itself in terms of how much it has acheived. I view it as an interesting side “good to have” feature to help with discovery in some marginal cases, but clearly not as the de facto obvious basis to create interoperated music stores networks.