Calories do matter (obviously, as energy intake is the entire point) but as you note the specific form that the fuel takes matters. However "carbs" is a catch all that includes plenty of things that (I assume) would be of similarly minimal use in this scenario. The calories need to take a very specific chemical form for this to work.

The wording is certainly confusing here, but yes the calories don’t matter as much as the form. Eating protein and fats simply give you minimal useful calories during the race. Even most carbs won’t be useful if they are more complex.

They're equivalent modulo some multiple. It doesn't matter which one we talk about, as long as we're consistent.

Then why replace one imprecise term with another? Fiber is a carbohydrate. Humans use close to nothing from its energy. (Though it plays another important role in the digesive system.)

Try eating 100g of grass per hour during a marathon and you will see. That's the metabolic edge horses have over humans.

Horses don't eat during races (and aren't evolutionarily disposed to marathons, anyway). No edge there; it takes quite a while for their symbiotic gut flora to downconvert fodder to glucose.

My gut flora won't do that, so they do have an edge. (Not during a marathon, but that wasn't my point anyway.)