From an interview with the incredible Bernard Melamud z"l, that I just came across:
The Fields: There has been much critical commentary concerning a statement you are alleged to have made: “All men are Jews.” Did you ever actually make this statement? Do you believe it is true? It is, of course, a view one cannot take literally. In any event, would you elaborate on the “All men are Jews” statement?
Malamud: I think I said, “All men are Jews except they don’t know it.” I doubt I expected anyone to take the statement literally. But I think it’s an understandable statement and a metaphoric way of indicating how history, sooner or later, treats all men.