But I don't think you have conclusively proven that Wrede never uttered those words. As Susan said: "we have one piece of evidence that is about as close to the event as one can expect to get against the biographer's version of events as given by an unstated source."
I disagree, the words cited by Roberts are very much disputed, I am not disagreeing that Wrede and Macdonald met, but as a good "historian" I have cross checked sources and couldn't find any confirmation about such a wording as cited by Roberts.
A good approach - in my opinion is - in case you state a hypothesis you have to proof it by checking before bringing it forward, otherwise I could claim anything - even the most rediculous statements and people would find it difficult to prove that those are wrong.
Without any doubt such wording makes good reading and selling.
Did you cross check Wrede on that?