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.

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.

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