Spotify app on the phone or tablet works, just not on the Roku which is not anyone else but Roku's problem since it's not supported only on the Roku
Roku does NOT write/code/maintain channel apps. The app’s provider does. Spotify has not provided video playback functionality in Spotify’s version of the app for the Roku platform. Each platform is a separate/different app altogther. Spotfity had write multiple versions of their app for each platform (android, iOS, PC, etc, etc). Not all versions have all the same features/functions. Similar to exclusive skins in xbox vs ps5 for the same game.