It seems that it is now able to use 3rd-party music sources on HomePod in iOS 14, according to this developer on Twitter:
Updated on 2020-09-25
A similar idea has also been suggested here, and only one thread can exist per idea:
Please vote for it, and add a comment there, so it may be considered. Any update on the matter should be posted there.
For more info on how the Ideas Board works, click here. 🙂