It is not a misapplication. It is impossible to retain weight if you are running a true calorie deficit. Biologically it just isn't so easy to run a calorie deficit as your body will slow your metabolism down accordingly. If your body is actually burning 3k calories a day and you are only eating 2 you will burn fat.

I mean, lets be sensible about this for a minute... We all agree that your body can not run off of zero energy (I hope)... Suggesting that your body will not lose weight on a calorie deficit not only suggests that it can run off of zero energy but that it is also capable of creating energy from nothing.

Also, I am a stone mason. My days involve 10-12 hours of constant aerobic and anaerobic exercise. I've had a fitness trainer and an ex-marine come out, the work was too hard for both. The fitness trainer quit after 3 days and the ex-marine 1 week.

My body at least has little trouble meeting my extremely intense energy requirements for long durations while in ketosis, I can't speak for others though. I hit no brick wall, although I do build them lol.

I should note that if I am not really careful about eating <20 grams of carbs per day I will get tired while working... Actually it presents itself in the form of dizziness and loss-of-balance after doing hard work for 20 minutes or so. I think that being in "really heavy ketosis" provides my body with as much energy as it needs. If I step up to even 30 grams a day I will start getting dizzy at work.