You are correct in your assumption. I recently asked this same exact query to Elan a little over a week ago:
Simply, all music must be identifiable using the (NEW) Plex Music agents and MATCHED in order to take advantage of the sharing mechanism, via share url OR the share image.
To reiterate, UNMATCHED music (albums, singles, etc.) that is more or less bootleg, white label, custom put together per your own method, will NOT be able to use the share url nor the share image feature.
