I tell you what PROVE me wrong. I'd be overjoyed to hear I could tell HomePod to play something via my Mac, not Apple Music. Please prove me wrong. Otherwise I'm going to go with what Apple and known Apple reviewers are saying which is what I'm repeating: Siri only works with Apple Music.
Again, put your money where your mouth is and show me a review where Siri played music from a Mac or iDevice. Otherwise you have nothing but your own imagination. Why would Apple want say it "only works with Apple Music," if saying that would reduce sales of HomePod? No, Apple wants to sell as many HomePods as possible and you don't do that by announcing before hand specific limitations that don't exist.