I can only conjecture.. the caffeine energizes something to help get s*it done. The empty stomach, something biological about, like when you’re full, your body is like ok I’m alive I’m satiated, don’t need to hunt. Coding is like hunting, you gotta kill the bugs so empty stomach is better.

My guess is that when we were hunter-gatherers, one of the strongest motivations for us to go out of the cave was an empty stomach. I am purposefully keeping myself a little hungry during working hours. The idea is that my body will keep sending signals to my brain to "not slack off".

digestion isnt zero cost

I can't think unless I have something in my stomach, so I'm sure it depends on your metabolism or gut bacteria or who knows what.