My understanding IS:
Hiit needs fuel FAST- so it burns sugar/glucose in the bloodstream. You will bog down once that fuel is used up. Which is your body telling you to slow down so that it can switch to fat burning.
If you eliminate/reduce your sugar intake, then low intensity, <120 heart beats/minute, exercise will help you burn stored fat. This is why walking is so good at reducing body fat.
Perhaps it's a good idea to warm up with Hiit to burn the sugar in your bloodstream faster so your body can switch to fat burning sooner.