And yet Rob Krar isn't into low-carb, is pushing 40 and runs the same distances faster... For that matter, the Bison Bar mentioned in the video you linked to contains roughly equal amounts of protein and sugar! Not that it matters much. Frankly, ultra runners can get away with more mistakes since there's so much less competition at their distances.

The top competitive distance runners (a group dominated by Kenyans and Ethiopians) generally eat over 70% of their calories from carbohydrates. Naoko Takahashi said she loved Korean barbecue. Bill Rodgers, the dominant marathoner of the previous genration, seemed to live off of pizza slathered in mayonnaise. Paula Radcliffe talked about eating a lot of fruits, yogurt and granola. While many, many runners have sworn by various odd diets, it doesn't look like anyone has been able to get a sizable edge from theirs.

I only raced 5-10k ranges, but I used to run greater than marathon distances once or twice per month in the mountains for fun so I can have some personal experience with "walls". Even a skinny person has enough fat on them to run for far longer distances than any race I've heard of. The "walls" are more about glycogen limits in your muscles. Extending the wall is mostly an issue of how high your aerobic threshold is, how well you can spare glycogen at the beginning (though this probably matters less for ultras) and your running efficiency.

All three of these factors are influenced by diet to some degree but training is by far the more important factor. Strangely, caffeine has a bigger effect than macro-nutrient composition.