This is dangerous nonsense. Is it possible to run ultra marathons on fat? Of course, you have enough fat stored for many many of those. Is it in any way amenable to a good performance? Not at all, and you don't need a biology PhD to realize that. The limiting factor in endurance sports is rate of oxygen uptake, fat metabolism takes more oxygen per energy yielded and is therefore strictly less efficient than deriving energy from glycogen.

Not to mention the majority of people will feel absolutely terrible trying to run in a depleted state. And there is no good reason to inflict that extra hardship on yourself when you can eat a large carb dinner and have enough glycogen to carry yourself through a half marathon

(Why can this guy still get good results? Ultra marathon is not at all a highly competitive sport. Someone to the far right of the genetic bell curve can handicap themselves with pure fat metabolism and still perform good. They would be absolutely killed in say the marathon distance where there is much higher selection pressure, if you want to call it that. Also, you don't run ultra distance at a high effort level, say 50-60% of VO2max - so here fat metabolism is naturally more pronounced anyway)