What else could it mean? Aren't "views" something that matches with "watch"? Maybe I'm missing something from how YouTube works, not a big user, happy to be corrected :)
It could mean that it marks the video as watched but without increasing the view counter. There are multiple signals that youtube had as a view counter or measuring what parts of a video were watched. Just because youtube knows a video was watched it may not increase the view counter to avoid bots or just because.
> Available options: [...] mark_watched: Mark videos watched (even with --simulate). Only for YouTube
https://github.com/yt-dlp/yt-dlp/blob/c8680b65f79cfeb23b342b...
So I'm guessing without that, it doesn't, so would have to add the flag for it to try to count it as a view.