you're technically right, but just nitpicking. Of course you could count the calories in fibers and that wouldn't make you fat, as your body can't process that. But most people do not eat fiber for 90% of their meals
I was hinting at something bigger than just a nitpick. I'm hypothesizing that gut flora interfere with macronutrient ingestion in a statistically significant way.
Microorganisms exist which produce glucose from materials that, if ingested by a human, would be emitted as waste.
What if colonies of these microorganisms were present in the human gut and unevenly distributed in the population?
CICO is a great approach for the general population, and works great even amongst professionals that have weight management as their career (bodybuilders).