Twenty years? ;-)
Perhaps I should believe that Mr. Thompson would admit to something like this if it is untrue, trying to cover for an anonymous source he could have just keep quiet over. Sure, he may have caught some heat from people saying "show us the money", but he decided to tell this story that calls into question his work for an extended period.
I don't think Mr. Thompson just didn't think about that while making up a story to lie his way out. I think he told the truth.
But, I would love to get more information to verify one way or another. If he is telling the truth, as I suspect, there is a bigger story yet to come out. And from the looks of things, it might be very big, given that we are already seeing tentacles heading towards Scott Ritter.