I believe that uses a separate database that was seeded from scraped youtube data but now it's only updated by those who use the addon. So it would have a much smaller sample size for recent activity.
I'm not sure about this because when going through Youtube shorts I regularly see videos with +100K dislikes, and any "viral" video has dozens of thousands of dislikes. I don't think this many users installed an extension to bring back dislikes and happened to dislike these particular videos.
The extension estimates the "true" dislike count based on the like/dislikes it has in its database. E.g. if the database contains 100 dislikes and 900 likes, but the video has a million views, it'll estimate 1000 dislikes. I'm sure there's some cleverness in there for videos where it doesn't have a tonne of information yet.