Unfortunately, it seems hard to imagine that with the current evidence we have today, a true rationalist could get farther than the position of "it seems very unlikely that there are significant mental differences between races", and yet that's essentially the right edge of the Overton window. If that's his reason for not discussing the topic, he can't go out and say it, because that's essentially like admitting he's outside of the window.

I'm not sure what you're saying here. Are you saying that the correct position is outside the window or inside it? (IMO we have pretty overwhelming evidence on all lines of inquiry that a certain position is correct, and that position happens to be quite outside of "civilized" discourse.)