he isn't following it long term, it is just a month experiment #1
#2 as humans we are the ultimate SCAVENGERS. this means our ancestors looked for ANYTHING we could eat. if we could only find meat then guess what? we ate meat. if we could only find fruit ? then we ate fruit.
#3 there were no freezers or fridges until the 1900's. our ancestors used salt/spices to preserve what they could, or they would smoke their meat (jerky) to make it last. but even those tactics did not really come around until the Roman age. before that you were lucky enough to get an animal you ate whatever you could and then that was it. you couldn't stick it in the fridge and reheat it the next day
the key is never stick to the same diet nonstop. there are seasons for a reason in nature. also every animal fasts from food here and there. even your own dogs/cats at home do it just by instinct.
the problem is you can literally walk into the supermarket and buy WHATEVER YOU WANT year round. and that itself is not natural