the non-scientific answer from me is that it's ridiculous. water fast, apple fast, whatever other stupid things are out there... idk who comes up with them. your body is an engine and it needs energy. i think you're looking for a quick fix or shortcut, and there just aren't any. you have to examine your diet, figure out your triggers, and avoid them. i'm sure alternativista will have a legit answer for you.
you may improve over those three days because you are inadvertently avoiding your dietary triggers, but then when you binge on the fourth day it'll be back to business as usual - you won't be healed. i do IF because i am cutting weight and it makes it easier for me. i can't comment on it's effects on acne for me, although i've read that it has benefits.
Well there is a lot of scientific evidence to support it from what I've read. I'm not an expert so I can't comment on how much of it is accurate or not but some doctors and scientists have claimed that it can even go so far as to cure diseases. I don't see why they would lie about it because water fasting is completely free, so it's not like they're trying to push a product or prescription.
But you're totally right about the diet thing, I'm definitely going to start improving my diet as well.