It is biased. As is any and all moderation, inherently.

Biased toward science. A bias toward the rational is a positive, healthy bias.

Rather than cowering about bias, Twitter should very openly and very aggressively stake out a clear and biased position; ie, this is our ground: we're pro science, pro vaccines, and we're going to hold that ground, period (whether anyone else likes it or not, and our algorithms will bias this way accordingly).