This video shows something that many youtube users don't know.
The channel owner can block another user from commenting on their channel. That most people know.
But in addition to this there is a variation where the channel owner can select "hide this user's comments on this channel"
Then what happens is that the person commenting sees their comment posted in the comments like everybody else. What they don't know is that no other user can see the comment.
So to them it looks like they are posting a comment that everybody can read. They see their comment up in the comment like everybody else's comments. However nobody else, not even the channel owner, can see this person's comment. They see no blank spot with the user's icon or any evidence at all a comment was made.
The only one who sees it is the person who commented. So they go on and on thinking that the channel owner is reading the comment and just choosing not respond but the channel owner doesn't even see it. The person commenting however thinks their comments have been posting normally. They may never realize this, making hundreds of comments nobody sees. Other websites also have this feature