Absolutely unexpected. A clarification though, which maybe you were aware of: the fasting period studied is no eating for 10 days straight, not intermittent fasting Ramadan-style. Given how counterintuitive this result is, I'm not sure I'd jump to the conclusion that intermittent fasting has the same effect.

Not really unexpected

Prolonged fasting is known to increase growth hormone production

Growth hormone promotes growth of bones and cartilage

Muslims that I know tend to gorge themselves every night for iftar. I would expect the body's reaction to be completely different than the 10 day fast studied here.

Unfortunately this is very true. The intention I feel is to rest the body during Ramadan, eat only what you need, maybe a bit less to actually feel hunger, become more spiritual, etc. But where I am, it is literally the opposite; people make huge feasts, with round two being sweets and dessert. EVERY SINGLE DAY. Add to that the lack of motion and activity and at 30 days we’ve put on 10 kg. Sorry to put this under your comment xD

That's not fasting. It's intermittent fasting: You never enter the fasting state.

Its not really unexpected. One could also think that building muscles during high caloric restriction diet/pure fasting is impossible but growth hormone works wonders for that.