"Casting to multi-room music from a line-in or Bluetooth input is not currently supported."
Yes, the analog line-in and bluetooth only work on the local unit. Supporting the analog line-in for casting to multiple rooms would would require an A to D in the device. In my system the multi-room music playback does work well with a spotify connect internet stream and there are room grouping options (controllable via voice or the Alexa App).