A couple of things about this. First, the first law of thermodynamics applies to closed systems which the body is not. Second, there is another option besides either burn calories or store them as fat; waste them. I find myself wondering why it is that these "learned experts" have not considered the possibility of that third option?
The body does not completely digest or burn all food that is ingested; a portion of it passes on through the body and is eliminated. The same thing goes for ketone bodies; if unneeded and unused, they are wasted through excretion (sweat, urine, respiration and feces) because they cannot be turned back into fat again and so the body has no further use for them. Feces, both animal and human, have been used in the past as fuel for fires. The energy is still there, but our bodies are not capable of utilizing it.
The human body is not a bomb calorimeter. I do wish all these experts would either realize that or stop trying to convince the rest of us that it is (or both).