This is from lean protein, add fat and your body is okay. You need either fat, carbs, or both. But you can’t just run on protein.

I thought the same, but an open question for me (now) is what happens if you eat 3000 calories of protein and 3000 calories of fat or even carbohydrate. You have sufficient calories that your body doesn't need to metabolize protein for energy, but what happens to all the excess protein you consumed?

I assumed the excess protein would be undigested, but can't back that up with a citation.

There should be enough fat in 12000 calories to avoid rabbit starvation. I did not consider the load of metabolizing all that protein and assumed the body would eliminate what it didn't need. They're might be something in protein toxicity here.