Greg, I do recall that Brokaw did eventually admit that Bentsen was blowing smoke — during the 1992 Democratic convention, on July 16:

It was Lloyd Bentsen who said to Dan Quayle `I knew John Kennedy, and you’re no John Kennedy.’ It was one of the electrifying moments of the campaign. At the Kennedy Library, just outside Boston, they went through all the files. They couldn’t see much evidence Lloyd Bentsen knew John Kennedy very well. But it certainly was an effective campaign ploy for him.

And who helped make that ploy so darned effective? That liberal media elite, Brokaw included.