I’d say you’re right on the money. Fasting converts stores of triacylglycerol into free fatty acids that the mitochondria can use for muscle energy (ketone bodies produced in the liver) while the glycerol moiety is converted into glycogen (by the liver aswell) and with the help of Glut transporter down regulation in the muscles and periphery tissue is made exclusively available to the CNS tissue (the blood sugar that is)
Now your brain needs roughly 200g of glucose per day to maintain homeostasis and keep an adequate membrane potential so your neurons can send signals through the entire length of the nervous system… but if your slim and fit there is simply not enough available TAG to be converted into energy and glucose so thats when the body starts to break down protein for gluconneogenesis but auto-phagy is not always the best state for our bodies long term but once in a while is okay.. but If there are no excess adipose cells to break down for the bodies protein needs You’ll soon notice hair stops growing and being unusually lethargic so thats when it’s time to re-feed and replenish your glycogen and protein.
I think it depends on how lean you are talking – 15% body fat is pretty lean and atheletic , but the body only needs about 5-7% fat essential. So there is still much fat (although not obvious like a big belly) for the person to be in fast; thinking of our hunter-gatherer ancestors , I would imagine they were pretty lean and there would be times whereby they went without feeding for a good while, and natural selection would favour those who are able to survive longer without feeding . So those lean people will also benefit from the autophagy and the benefits a fast give.