From Dr Fung's presentations, interviews etc, I've gleaned that his fasting protocols are aimed at individuals with obesity coupled with chronic illness (usually diabetes). I recall at least two interviews where he essentially communicated that his protocols aren't meant for those who don't have any/much weight to lose. And I've never seen any fasting-for-mass themed material from him.
He uses improved bio-markers, how many/much medication(s) he gets his patients off and probably weight loss to gauge whether his intervention is working. The protocols themselves extend from IF to block fasts of various intervals up to 7 days. And in response to a specific podcast question, he recommended Keto during the IF eating window.
The table of contents of his book has a 'Who Should Not Fast' chapter but, curiously for me, no 'How to Maintain Weight Loss' chapter. I've also searched his blog for specific information about minimising lean mass loss and avoiding nutritional deficiencies and found his responses interesting but unconvincing. All told, modified fasts like RFL are a safer bet.