Give a healthy person insulin injections and they will gain fat (and muscle) on the same calories. It's basically what modern bodybuilding revolves around. Diabetics are not healthy, they do not produce enough insulin so it must be administered exogenously to try to mimic healthy levels, that's why they shouldn't get fat. People commonly reverse diabetes entirely by eating zero carb diets.  As an aside it's common for bodybuilders to gain muscle while on a caloric deficit while using other hormones. I have personally gained 5lbs of muscle while on a starvation diet while using trenbolone. This should be a violation of physics according to some Einsteins on the internet. Reducing the entire subject of metabolism to calories in calories out is foolish. The second law of thermodynamics applies to closed systems and the human body is not a closed system. It usually holds true, while hormones are within normal ranges, but it's easy to point extreme scenarios where CICO falls apart as I have done. I recommend this talk by Dr Ben Bikman which goes into detail about the caloric vs endocrine models: https://www.youtube.com/watch?v=8t1JN0RgvO4