> If I eat 1000 calories of whole oats, a lot of those calories are passing through my body.

Humans are extremely efficient at extracting energy from food, regardless of the fiber content. We owe our survival as a species to it, as food was much less plentiful in the ancestral environment.

I've heard of fiber meaningfully affecting insulin response to food, satiety, and some other things - perhaps even gut microbiomes - but not metabolic efficiency.

Yeah he's posting nonsense. If it were true you'd be passing huge chunks of undigested food and be sick. Very little of the calorie content of oats and other so-called fiber rich food is actually undigestible. Sorry...beating obesity is not as easy as eating cereal and oats all day. wish it were.