The dietary fat that goes into structure isn't all "essential."

A common criticism of the low carb diet is that de novo fatty acid synthesis doesn't seem to be very active in humans, except in situations of extreme carbohydrate overfeeding. This is a straw man, carbs don't have to be made into fat to make you fat, they only have to prevent the oxidation of fat. Animals that naturally live on a high carb diet tend to be pretty good at synthesizing fat from carb; it's easy to measure in rodents, hard in man, although this may have more to do with rate of fat turnover than anything else.

But, given the difference in blood lipids between a very high fat diet and a very high carb diet, I think that it's fair to say that there is a very big difference in metabolism between the diets, and that there are important differences between the two that go far beyond considerations of vitamin absorption, essential fatty acids, and energy needs.

That bit about 30 or 35 percent fat intake depending on activity level is pretty hilarious. Good luck finding a study where that kind of difference had any sort of measurable effect on anybody.