As far as I read through how Spotify works, you cannot exclude their official app when you want to cast playback to another device (Spotify Premium feature) because the authentication and authorization between you and Spotify servers are done exclusively by the app, and when you select from within their app your Linkplays speaker, it only gets a temporary authentication passed over by the app to the speaker, so the speaker is able to download contents directly without the app. And thus the linkplay speaker is able to forward metadata via UPnP to Home Assistant.
If App disappears from this environment, after a while Linkplay’s authorization will expire and app is needed again to activate it. This is how Spotify works and it’s their intention to make users hard to exclude their official apps.
I also tried HA’s Spotify integration but I’ve found it to be limited in usage and it cannot replace the official app just because of the above. It will behave pretty much like another Linkplay device - in regards to the Spotify API. You still need Spotify’s official app to use HA’s Spotify integration nevertheless.