I suspect their dataset was direct link dislikes. I.e. people who arrived at YouTube from another site at a specific video who dislike the video. That's the characteristic of a coordinated dislike campaign.

> the feature feeling useless, as even if you dislike, you might thing that you click will worth nothing as no one will see it

That was exactly the point. They didn't want the attack mobs seeing the fruits of their labor, making it feel useless, making fewer people mob.