It's very likely that it was most of it. It's just how fasting works. Your body enters ketosis in a day or two and uses fat for fuel. Your body would have to be rather dumb to burn muscle at that point.

I don't have a good way to measure that, but if you have one let me know.

I can say that after all this, I was climbing hard 5.10s and easy 5.11s, and I'm 40, and I haven't been able to climb stuff like that since I was about 22.