[thaumasiotes]

What is the claim you're making? Assume 80% of Example Man's diet is his rest energy requirement. The other 20% goes toward discretionary things like moving. This means that if Example Man doubles his discretionary activity, his food consumption will increase by 20% rather than 100%.

It does not mean that if walking three miles takes one apple worth of energy, Example Man actually only needs to consume 0.2 apples in order to walk an extra three miles. He needs to eat the full one apple. This article doesn't consider food as a percentage of your diet at all; it considers food as the cost of moving. Your point isn't relevant.