Also weren't most of the questions that the TQP lawyer asked ad hominem? I understand that he asked those to discredit Diffie, but they still seemed very disrespectful. Almost anyone in the crypto field would back up Diffie's reputation, so that was a rather stupid move on the part of the TQP lawyer IMO.
Ad hominem is when you attack a logical argument based on the characteristics of the person making it. It's a logical fallacy because the logical argument is something that can be evaluated itself using logical reasoning.
But an expert witness does not present a logical argument that the jury can evaluate on its own merits. An expert witness presents an opinion and explains the evidence on which he relied to form that opinion. Facts that credit or discredit the witness help the jury determine whether to believe the opinion.
Incidentally, this is why the Federal Rules of Evidence are a better basis for internet arguments than the logical fallacies. These are rarely a matter of reasoning, but rather a matter of assessing the credibility of opinions. When you argue with someone about global warming, you're not presenting logical arguments that can be evaluated standing alone. You're arguing over which experts have more credible opinions and which sets of evidence you believe.
Unfortunately, the jury is very much not in the crypto field. As far as they know, he's just some guy who claimed to invent something, and the plaintiffs are making the most of it.
Similarly, aside from the ad homs about educational pedigree, the line about not being the "real" inventor is pretty critical. The point is "secret" (i.e. unpublished) inventions don't count as prior art in invalidity arguments. So even if you accept that GCHQ invented public key crypto, Diffie's patent is still valid. I imagine IP Nav wants to argue that private demos of Lotus Notes don't count as prior art.
Attacking the credentials of a supposed expert witness is a perfectly legitimate strategy in court. He wouldn't really be representing his client well if he didn't try to discredit Diffie.