A system running at its limits is more sensitive to fluctuations in inputs.

That's as true for your body as it is for a nuclear reactor.

Yes, you will certainly be aware of what it takes to keep running at that rythm. But that's not neccesarilly what is healthy.

So you accept that regular exercise is useful for understanding the physiological implications of the diet, but you question whether regular exercise is healthy?

No. I believe there is such a thing as excessive exercise, which I feel is what the post was describing. While under and excessive excessive your body will be stressed enough that it should be easy to see the effects of your diet, and so it would be relatively easy to optimize your diet for such a regime. But the diet that such a regime requires is not necessarily optimal (or good) for people with a more reasonable and healthy lifestyle.

What does this even mean? This is such an huge generalization that it seems it could be true or false depending on what example you pick.

Sounds like a justification for performance enhancing drugs.