that's not true about your theory on fat gain from insulin spike.

its called anabolic window which u already know about, at that time you are in a catabolic state(muscle tear down) simples carbs(sugar) consumed with fast acting protein, gets you out of catabolic and into anti catabolic that's your GOAL (quick as possible cutting or bulking), and that's why you consume solid food(complex carbs) later on, since it takes a good hour and half to fully digest and use nurients. when you train a muscle it will act like a sponge to the pwo simple carb/protein drink, and u need to get the accurate number of carbs so there isnt any overspill of unwanted simple carbs that would contribute to fat gain