The HomePod can act as audio out for any Mac/iOS device. The music app on Mac can also stream directly to HomePod. (Or you can send system audio to it as well)
Correct, but can I have Music open on a Mac with a shared library and then say to the HomePod "Play ZZY" will it be able to locate that on the share and play it? If I cannot talk to it and have it find the shared music then what is the point?