Perhaps it's compatible with iTunes Match and the uploaded music that wasn't matched will be streamable to HomePod regardless.
With respect to whether or not HomePod can be used to play music in your iTunes library, a couple of additional points:
- in Apple's own description of Apple Music, they include the following statements:
--- "45 million songs. Plus your entire iTunes library. Apple Music allows you to stream our catalog of 45 million songs, along with everything in your iTunes library - no matter where it came from."
- In Apple's description of iTunes, it includes the following statement:
--- "All the music in your personal iTunes library — no matter where it came from — lives right alongside the Apple Music catalog"
- In the Apple iTunes Support section, under "Access your music collection on all of your devices with Apple Music", it includes the following:
--- "What happens to the music you already had on your devices when you join Apple Music?
--- When you first join Apple Music, we identify the songs that are on your iOS device or computer and compare them to the Apple Music catalog.
------- Songs that you bought from the iTunes Store are automatically available in your Apple Music library as long as you use the same Apple ID with Apple Music that you used to buy the music.
------- If we have your version of a song in the Apple Music catalog, we make it instantly available to access on all of your devices for the duration of your membership.
------- If we can’t match certain songs in your collection to songs in the Apple Music catalog, use iTunes on a Mac or PC to upload a copy to iCloud Music Library. After the upload, you can access it on all of your devices.
--- If you have music on your iOS device or computer from sources other than Apple Music or the iTunes Store
------- You can have up to 100,000 songs in your music library. Songs that you buy or bought from the iTunes Store don't count against this limit.
------- Only songs smaller than 200 MB or shorter than 2 hours are added to iCloud Music Library.
------- We create AAC 256Kbps versions of songs that are encoded in ALAC, WAV, or AIFF formats and add the AAC version to iCloud Music Library. Your original music file remains untouched on your Mac or PC.
------- We won't add songs to iCloud Music Library that are encoded in AAC or MP3 formats and don't meet certain quality criteria.
------- You must authorize songs that contain Digital Rights Management (DRM) for playback on your Mac or PC before we can add them to iCloud Music Library."
--- Finally, in Apple Support under "Download iTunes 12.7.3" (posted Jan 23, 2018), it states: "iTunes is now designed to work with HomePod. Use the improved AirPlay menu to easily choose HomePod and control what plays next with your Apple Music subscription."
So iTunes works with Apple Music to give you access to everything you own, and the process is similar to iTunes Match in that a copy of your personal library is created in the iCloud Music Library and you access it from there when you're using any of your devices. And iTunes can stream to the HomePod via AirPlay (current AirPlay, not just forthcoming AirPlay 2).
If you are at home and, for example, using your iMac to stream music to your HomePod, then it will be using iTunes on your computer to stream music to the HomePod. It just seems to me that in this particular circumstance, it wouldn't make sense for music that you have existing on your computers hard drive to be ignored and iTunes would rather use the copy that had been created in iCloud Music for streaming to the HomePod. Would it?
I am still not entirely confident that HomePod can be used to stream music from iTunes, i.e. CDs ripped into a personal iTunes library, without having Apple Music involved, but it sure seems that the capability is there, or should be there, even if it is not explicitly stated in the HomePod descriptions currently provided.