The scenario described in the OP does not involve commenting on a private video. It involves commenting on any public video, then the uploader clicks on a suggested prompt in YouTube Studio which supposedly processes the comment and creates a URL with the title of a different video.
Okay yeah that was my best guess also, thanks for confirming. I don't know the modern yt back-end well enough to understand how it would mix these things up but it indeed can't work otherwise
