I believe you're onto the right idea... Spotify holds out Spotify Connect as a premium-level featur. If I'm remembering correctly, they developed this ahead of Google Cast Audio being released. So it's not as though Spotify had bad intentions; an audio casting protocol was needed for Spotify users.
The current sin seems to be that Spotify clings to Connect potocols/stack/whatever as the method of connection to Google Cast devices. They could, I presume, fall over to Google Cast APIs, and we'd have the same good casting experience that Google Music app has. but they don't, and we're stuck with the current implementation (even on beta) where casting to many Chromecast Audio devices simply doesn't work. Get it together Spotify!
Everything I want is on Google Music, even more so than Spotify actually. (Also you can upload thousands of your own tracks for free to Google Music.) But, finding what you want is awful on G Music. The iOS and Android interface is so big, so much white space, so weird...I just can't even
It doesn't seem to have anything to do with Google. It's all on Spotify, I suspect. Spotify just needs to suck it up and use the publicly published Google Cast standards instead of whatever mess they're running. For some reason—which I think involves Spotify Connect—they haven't/won't to date.