It's not just made up, it tracks downvotes from extension users and combines them with a historical database. And I think some creators share new info with the database to help the accuracy.
Many videos had the dislike count collected by the extension and stored before YouTube completely removed the counter from the videos. That number is now combined with dislikes sent by the users.
New videos will only show dislikes sent by people using the extension. There's no way to know the real number anymore.
I don't think many people are going to go through the trouble of offering to help and then feed in fake info, especially if they make awful videos and it's easy to guess what they're doing.