Blueberries consist mostly of water(proteins, carbs and fat make total of 7.5 grams per 100 grams, more or less rest is water), so they aren't very rich in calories(44 kcal per 100 grams). Of course all sorts of berries would rather be an supplement than a primary energy source in the first place.

But indeed, kilograms per day? Sounds too much to be optimal, and relevancy for aging or combating it from perspective of brain development is totally left out, which makes the whole claim even more astounding. Sources or gtfo! :)