They showed the cap was 2.5 times the body’s resting metabolic rate, or 4,000 calories a day for an average person.
Anything higher than that was not sustainable in the long term.
So when you go and read the actual paper, you find out what “long term” actually means, and it’s basically a full year. It’s a logarithmic curve asymptotic at 2.5xBMR. If you’re going for a shorter time frame, like say a measly 100 days, you can still crank out around 4xBMR.