how do you know?

This sounds like scientism.

Because I have experience with good rationalists, and the kind of people they have relationships with, and I am a bayesian so I can assign degrees of belief to propositions that I haven't tested directly. In this case, it seems reasonable that similar people have similar relationship-behaviors, and so my existing knowledge is relevant.

Rather like "how do you know that the fastest dog in the world can't outrun a formula one car?" - I know this with high certainty because I believe that similar animals behave in similar ways.