Can you specify what leverage you think Brubeck has over an independent professor's career in order to make threats?

I see none, and consequently Brubeck's explanation makes more sense to me. I understand he meant these words, which he supposedly retracted on the spot, in a "why would you ruin your career with this behavior / turning down the opportunity I am offering" way.

You don't think that is the likely explanation?

Fair enough then. I had only seen some earlier comments.