Even if Apple didn't have a competitive offering to Spotify, it'd be a terrible idea to adopt a third party's standard/SDK that impacted such a core part of the ecosystem and UX as controlling and handing off audio. The reason people buy into ecosystems is so there's a single entity that is weighing and making decisions about what creates the best user experience, which necessarily means having control over the roadmap of standards and interfaces.
Even if we were designing the perfect tech ecosystem that was free of business or competitive constraints, it still wouldn't be a good idea to have an app developer, with their own motivations that may not be aligned to users, dictate SDKs and standards to ecosystems or platforms. It's a weird and unsustainable power dynamic that misaligns incentives. It'd only be viable if in the process the app developer opened the standard and shared control of its future.
As much as Spotify wants to be a platform, what they offer is just a service, and they're not positioned to be controlling such standards. I love Spotify and much prefer it to Apple Music, but what they're doing is choosing not to build features that their customers want on platforms their customers have chosen in order to gain leverage in the regulatory battle over the 30% cut. I respect the desire to have a more level playing field but their other complaints are disingenuous and are not in the best interest of their customers.