The word 'guarantee' requires two things to happen in order to be of any value: One is a wager by which you lose something if you are wrong. The other one is the ability to check the outcome of the wager. Neither condition applies and therefore you cannot guarantee anything.
I do not know if Wittenberg sold the stock. Unless you have information that is not available to the public, you do not know either and therefore any guarantee one way or the other is pure BS.
I have the impression that your opinion of Wittenberg was not very good and therefore you should welcome his resignation. Otherwise you would fall into a contradiction: "We do not want him in the Board but we do not want him to leave".
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