Hello, Mr. Verner - a thrill to have you here with us. Hello, Doug.
Well, thanks for informing us of the jargon. I thought that was all there was to it. You can question what might have happened (and write your own book, as Derek says) but, even if you don't trust Derek's memory, you can't change what he remembers.
However small and unimportant this detail is, I can think of things my own father said when I was 8, that he has long forgotten because they were not important to him. Yet, as a kid they impressed me in some way and I remember the wording so distinctly that I would not let anyone change it.
Here is a theory. What if Doc tilted his head, as if pointing to himself, in complicity with the guy, while uttering "mmm..." and then continued: "mmmwith it." Or maybe he did utter the "I" because he was going to say something else and suddenly turned it into: "I....mwithit."
Being the subtle magician he certainly was, it may not be too far-fetched to think that he had to get the guy's attention before casually "withiting" him.
Another theory is that he said whatever he thought was right and got away... with it.