We have code that tries to stop users from adding a CD stub while logged in (including an entire TT template for that) but we're never showing it because of a bug, meaning you actually can just add a CD stub while logged in just fine (e.g. https://beta.musicbrainz.org/cdstub/add?toc=1+11+243566+182+19677+40555+64800+86005+109520+131830+154965+173392+194120+218614 should give an error)