Agreed on this point. Fasted low intensity probably won't cannibalize your muscle at all, i.e. a light walk.
If we're talking about a fast jog, running, HIIT, sprints etc, I personally would rather side with caution and if trying to lean up, use something like MAG-10, aminos or worst case 15-25 grams of whey isolate for some insurance.
Insulin is simply the transporter that shuttles nutrients, it doesn't blunt fat loss just because it's present. Blunting fat loss would require being in a surplus of calories or the insulin being accompanied by the presence of a high amount of glucose/carbs.
Everyone has to understand that in order for fat to be utilized as a direct energy source, there has to be a high presence of oxygen and also time, which is why fat is our primary source of energy utilized while we sleep.
During something that's high intensity, fat cannot be a direct energy source because as soon as the activity goes anaerobic and energy is needed immediately, our body switches our system towards blood glucose that's present and once that's depleted glycogen.