It is surprisingly close. I download a lot of videos with yt-dlp and have seen ratios where up to 20% of total likes being from Return YouTube Dislike users.

Like here's a video with 188,573 likes in total of which 28,330 were submitted to the data base:

https://returnyoutubedislikeapi.com/votes?videoId=dtjb2OhEQcU

The users disliked 130 times which was extrapolated to 356. No idea how that formula works, I believe users with this plugin could be more likely to dislike videos than users without that plugin which may be factored in.

And they have data for older videos from before the button was disabled so those are accurate