No idea why the people in this comment thread get downvoted, but the extension is literally not an actual, factually correct count of youtube-wide dislikes, and the creators say so themselves.
From their own homepage:
“The backend is using archived data from when the youtube api was still returning the dislike count, extension users like/dislike count and extrapolation.” (www.returnyoutubedislike.com/faq)
It’s a mix of outdated pre-removal data, dislikes of only(!) the users of the extension, and guesswork. Dislikes of youtubers who do not use the extension are not factored in, and “extrapolation” is a fancy way to describe that part of the data is made up based on assumptions.