> If you don’t know that material enough to talk about it in detail impromptu then you have no business giving talks about that topic.

I don't think this follows. There are many things I am an expert in that I'm not an expert in communicating about impromptu- I've been to many talks where the speaker is clearly an expert but was not prepared to speak.

I certainly think there are people for who expertise == ability to speak about it, but that's certainly not always or usually the case.