Muslce loss? Sure, that is if you don't eat enough calories at the end of a fast or make up for it the on the following 'feed' day. I can certainly see this being an issue on the Warrior Diet. Too few calories.
However, with an IF schedule most follow here, enough calories are consumed after/before the 20 hour fast.
It really depends on your total calorie intake at the end of the day. Every other day I fast until 5 pm. 5 pm on, I'll have 2-3 meals giving me a total of around 19-20 blocks. If I fall short, I simply make up for it the next 'feed' day.
Not sure how you would make that work on the warrior diet? I would think 20 blocks in one GIANT meal would be nearly impossible (not like I was ever that brave to try it anyway).
100,000 generations of humans have been hunters and gatherers; 500 generations have been agriculturalists; ten have lived in the industrial age; and only one has been exposed to the world of computers.