It's hard to take Spotify's call for openness seriously when they're just as controlling as Apple is when it comes to how it allows its users to use its product.
Most egregiously, they never delivered the promised playback-capable replacement for libspotify, probably because they want to keep their users in their little walled garden where Spotify can subject them to incessant A/B test tweaks to the UI designed to herd them towards the most profitable content. Third party clients that just act like normal music players and don't change constantly would be an affront to that.
As you've pointed out, when Apple has made affordances to allow better support for third party services, Spotify has been slow to take advantage of them. HomePods have had APIs that third party music streaming services can hook as well as the capability to set your default streaming service to things that aren't Apple Music for a year and a half now, and as of this writing Spotify has not used them, despite smaller streaming services like Deezer and Pandora having implemented support months ago. Why? I can only guess because HomePods aren't Spotify Connect devices and thus aren't under the control of Spotify the way that Spotify Connect devices are.
Spotify isn't upset by the lack of openness, they're upset that Apple won't give them the same VIP treatment that they've gotten from other platform/device vendors (including Microsoft, with Spotify being bundled with Windows 11).