Well he's right in one respect, but "the yanks" have played no greater a part in all of this other than being the largest off-road market in the world. If you want you could then argue by default that they've played a greater part but it would be a very mute point indeed.
However, his overall analysis of the situation is absolutely spot on but again take just about anything you want and the same theory can be applied. Watch a modern GP with their huge run off areas, etc, and then look at where they were racing 30 odd years ago and you'll find a similar thing. Now they have traction control, fly-by-wire, fuel maps to curb the power delivery, back then the quest for power delivered some of the most violent machines ever produced.
Can we blame the Japs, well a bit like the America thing, they were the guys who produced most of the bikes over this era, but yet again it was people the world over who bought the stuff (including Europe) and unlike now when you can absolutely lay the blame for this present 4-stroke debacle at the Japanese manufacturers in general and Honda in particular, I doubt there was similar deliberate manipulation of the situation back then.
So overall, his assessment of things is correct but his attempt to attach blame is where he gets it wrong.