What you are saying, unfortunately, is incorrect.
Spotify allows for third-party control and reading and changing of their database. In fact they’ve made this very easy to do… Have a look here https://developer.spotify.com/
I got my Home Assistant hooked up and this works amazing. I can start/stop etc… and even stream directly to Spotify Connect enabled devices from within Home Assistant. I can browse through my playlist from within Home Assistant (a free open-source project for home automation). Even the free Volumio project has Spotify fully supported. See the Home Assistant docs how easy this is. https://www.home-assistant.io/integrations/spotify/
So I think we have to turn this around and ask a different question. Is Roon willing to add support for Spotify? I say not