I don’t get the same results in the Sonos app when searching Storytel compared to the same search in the native Storytel app. I have also seen similar results with Spotify. Something can be found in the native music service app, but not when searching for the same thing through the Sonos app.
E.g. I searched for an author in the Storytel app. I got 59 results. Searching for the same author in the Sonos app just gave me 38 results.
I can of course “cast” from the native app to my Sonos system and start playing it that way. But the queue functionality is very limited when doing it this way and it is not possible to queue multiple things in this manner.