What does that mean?? If I use Spotify, it will send the audio output to either the CC Audio only or the normal CC including cover art, navigation bars etc, so clearly it is possible!
It means exactly what I said. The support for Chromecast is only for the display of information, not the playback of audio. Since Chromecast Audio has no display, it is not supported by the app. Unfortunately since Android does not differentiate between the normal and audio only devices, the app can't specify that it requires a Chromecast with a display.
When you use Spotify, it is not sending audio from the device to the Chromecast. It is just telling the Chromecast to stream audio directly from the Spotify server. If you need proof that this is how the Chromecast is designed to work, just look at the Google music app. It can't send audio files from your phone to the Chromecast, it can only stream from the cloud.
There is a complicated method an app could stream from the device, involving creating a music server on the device and telling the Chromecast to stream from that instead of the cloud. However Squeeze Ctrl is NOT a music player. It is just a remote control, it has no access to the music stream at all. If I was to do something like that, I would put it in SB Player, but I don't really think it's worth the effort.
Also please don't tell me to use the built-in mp3 stream that LMS provides. If I add any sort of Chromecast Audio support, users will expect to get full Squeezebox functionality and that stream cannot provide that.