I don't have time to watch it today but maybe will later tonight since I have 3 weeks off. But if that is what he says, in general he is right. Metabolically, insulin promotes synthesis of fat (fatty acids and triglycerides) and impairs mitochondrial beta-oxidation of fatty acids. That being said, there are certainly a lot of BB'er I know who have used insulin while cutting and gotten good result, still dropped fat and managed to retain muscle. I have never done this but the key seems to be keeping your carb intake around when you do the fast acting insulin, GH/testosterone.
https://pubmed.ncbi.nlm.nih.gov/1641767/ But then we are spealking of chemically enhanced athletes. In a BB when you combine carbs with insulin, GH and testosterone the carb and amino acids are immediately shuttled into the muscle, thus supersaturating the muscle causing sacoplasmic hypertrophy which kick starts myofibrillar growth and not fat storage. At this point you are already taking in very few carbs.
Again, I warn about putting too much stock into medical doctors who are practicing nutrition. Most only have 1 undergraduate nutrition course under their belt. Their expertise is medicine and in the case of Dr. Jason Fung he even says he is a Canadian nephrologist. Why would a medical doctor who specializes in kidney issues have a web site claiming to be the "diet doctor"? Kidney doctors here in Houston make plenty of money and have no time to play nutritionist on Youtube and Instagram. If they feel you need one they refer yout to a reputable nutritionists. Then in the next sentence Dr. Fung claims he is the world's leading expert.....intermittant fasting, low carb and type 2 diabetes. Yet his speciality is kidney issues, nephrology? I do know guys who are experts in all three fields but they don;'t clam to be medical doctors rather PhD's in human metabolism. Want some good information on intermittant fastin, type II diabetesg and low carb/ketosis try Dr. Stephen Phinney and Dr. Jeff Volek. These two IMHO are true experts in the field.
Watch this excellent talk by Dr. Phinney and Dr. Volek, two esteemed researchers, on the science of ketosis and keto-adaptation.
www.virtahealth.com