is iTunes Match a substitute for backing up iTunes music?

In my opinion, no. The matching process isn't guaranteed to be perfect, so you can potentially find that iTunes has a slightly different version of a track compared to your original. Uploaded content might also not be redownload in precisely the same format and quality that was uploaded. Lastly your library may have music that is ineligible for iTunes Match, along with non-music content that isn't captured in your iCloud Music Library. I prefer to consider iTunes Match as a library syncing service, that may also act as a reserve backup in extremis. See Backup your iTunes for Windows library with SyncToy - Apple Community for my suggested approach to backup.

tt2