Apple’s updated the specs page for the HomePod to clarify which audio sources you can play music from:
iTunes Music Purchases
iCloud Music Library with an Apple Music or iTunes Match subscription
Beats 1 Live Radio
AirPlay other content to HomePod from iPhone, iPad, iPod touch, Apple TV, and Mac
Great that the company finally spelled it out, though it really should have provided this information both earlier and in a clearer fashion.