I feel like this is greatly exaggerated. I did half of that in 3 days (500km) with the last day being 250km (because I was sick of doing it) and I didn't feel any such "caloric furnace". I ate a couple of pbjs each day and then had some rice and beans for dinner. I didn't do this very intentionally, I just didn't have much choice (I did this ride in Africa, where there aren't many ice cream shops).

It's universally understood that a century is one of the easiest "feats of endurance", which is why you see people far into their 50s and 60s still able to do them.