I think he's wrong equivocating the invalidity defense with the prior art defense. My understanding is a patent can be invalidated - and rendered completely ineffective - if you can show that it doesn't actually teach a practicable implementation of a way to achieve the claims.
I had experience with this. We received a cease and desist letter from a (large) company saying we were infringing a patent they had claiming synchronizing audio playback with the movement of a cursor. After carefully reading the description, we realized that they were actually describing doing this synchronization by assuming that the real-time clock signal was all that you needed to know how much of the wave file had been sent to the audio output ... and we knew that this could not actually work. It didn't account for processing delays owing to CPU/memory/bandwidth limitations. Our lawyer wrote a letter back to them saying this and we never heard from them again.
Note that the _claims_ themselves did not describe the synchronization method - they were claiming the generality of doing the synchronization. It was in the _description_ that they explained _how_ to do the synchronization and this is where we found the flaw which invalidated the entire patent. I should note also that the description included words indicating that the method they were describing was "essential" to the invention - so it was actually a badly written patent. If they had carefully qualified the description with words like "this is one possible method ... there are others known to those skilled in the arts", we might not have been able to make this defense. And, of course, this never went to court (probably because they realized how badly the description had been written). But, I've seen other such flaws in patent descriptions - you'd be surprised how often lawyers make stupid mistakes like this.
And, if you do find such a mistake, you will have helped to move toward invalidating the entire patent, as opposed to just avoiding the particular infringement suit. It is lots more work to wade through entire descriptions, and I wouldn't recommend doing it unless, as the speaker indicates, you are in the cross-hairs of an infringement suit. But, it can be a very good feeling if you succeed!