No, Apple doesn't do that at all. What Apple has been doing is adding Siri functions in "groups". They're not "choosing" which Apps specifically can work or not with Siri.
This has been going on since iOS 10 with each new version of iOS adding even more Siri functionality for third party Apps. iOS 13 adds functions specifically for music, radio, podcasts and other media related items. That's 4 years and 4 versions of iOS with expanded Siri functionality.
Fair enough, but Spotify is a pretty popular app that if it didn't directly compete with an Apple service probably would have been one of the first to have been permitted to use Siri. I'm sure there are way less downloaded/used apps that have Siri functionality already. However smart for Apple to use the slow roll out of 3rd party Siri support as good/legit excuse for taking their time to let Spotify have party Siri support.