Pretty much spot on. I believe they also saved the dislike counts on lots of videos from before the dislike removal, so it's only new dislikes not reflected accurately

Also, you can help fix the bias! The more people using the extension and disliking things they for like, the more accurate it'll become

(There were also plans for some kind of API that would link to YouTube accounts to allow content creators to feed the true dislike counts into it constantly, but I believe they were abandoned due to privacy and safety concerns as it required a lot of permissions and essentially root level access into your account to function)