How would the first contacted Trisolaran keep the secret of first contact?

So when Ye Wenjie contacts them for the first time and the Trisolaran who responds says he is a pacifist unlike most of his race and that she should not contact them again or the others would come to conquer them. The issue I see with this is the Trisolarans are supposed to know everything about one another, no lies no secrets. So how would it even be possible for that one single Trisolaran to keep the secret of first contact?

Is this a plot hole or am I missing something?

Thanks!