Most of the people that get to that level are bodybuilding competitors. If the population weren't specified, I'd agree with your suspicion, since it is pretty well demonstrated that most weight loss issues are related to inaccurate food reporting/consumption. However, folks that are serious about bodybuilding are pretty meticulous about following diet plans, and aren't strangers to hard work, denying physical urges, and doing really uncomfortable stuff for their sport. I'd agree that a typical obese person is almost certainly just underreporting their calorie intake.
There is no metabolic ward evidence for this, if that's the standard you're after. Layne hasn't had anyone do this sort of routine because he thinks its incredibly harmful for long term success in bodybuilding, so he hasn't personally seen this. However, he has coached those whose prior coaches did this, and he worked to repair the metabolism.
Respectfully, that's not even strong evidence, let alone extraordinary evidence. It's just hearsay.
I would be willing to bet serious money that any healthy adult, carefully monitored, who did 2-3 hours of cardio a day and ate only 800 to 1000 calories would either lose weight or (far more likely) end the experiment because because he literally felt like he was starving to death.
By the way, I was reading the other day about a study where professional nutritionists were assessed to see how well they reported their calories. And they under-reported significantly.
Eliezer and I have put together this first pass at a recipe for DIY ketogenic soylent--or, as he prefers to call it,The Mildly Surprising Super Ketonic Dietary Replacement Weight-Loss Fluid - It's Not Food, It's Dietary Replacement Fluid!(R) (I am not in full support of this particular preference...)
So let's play Make a Prediction. It seems more likely to be useful than "free associate with why soylent/ketosis is awesome/stupid". Imagine that Eliezer has around a week's worth of infinite willpower (that can only be spent on resisting food cravings etc.). Further, imagine your crystal ball shows you that a month from now, he hasn't lost any weight. What does it tell you about why?
Ketogenic Soylent v1
1 scoop Gold Standard whey protein
1½ tsp calcium citrate powder
1 tsp creatine
½ cup+1 tbsp ground flax
4 tbsp Hershey’s Special Dark cocoa powder
2 tbsp lecithin
⅗ tsp potassium citrate on day 1, increase gradually to 1tsp by day 5, don’t exceed 2tsp
½ tsp iodized salt
⅛ packet Emergen-C
⅕ cup olive oil
1¼ tbsp flaxseed oil
⅗ cup MCT oil
Sucralose to taste
We might need to add water to get the consistency right.
Pills
1 capsule vitamin D supplement/day
2 Opti-Women vitamins per day
1 capsule MSM
Totals Per Day
Calories: 1866
Fats: 193.5g
Carbs: 12g
Protein: 54g
Fiber: 25g
Biotin 250mcg
Calcium 150mg
Chromium 120mcg
Copper 2mg
Folic Acid 600mcg
Iodine 200mcg
Iron 18mg
Magnesium 75mg
Manganese 5mg
Methyl-Sulfonyl-Methane 1000mg
Molybdenum 70mcg
Niacin 20mg
Pantothenic Acid 20mg
Potassium: 5g
Riboflavin 20mg
Selenium 70mcg
Sodium 3g
Thiamin 20mg
Vitamin A 5,000IU
Vitamin B6 20mg
Vitamin B12 100mcg
Vitamin C 250mg
Vitamin D 1600IU
Vitamin E 100IU
Vitamin K 80mcg
Zinc 15mg