Actually, there were studies that say it depletes some B vitamins. And if they don't eat what you give them, you must throw it out. So, there is waste with this food as well. I do not believe it saves feed; it may only in the fact that YOU choose to feed them less because you throw down their daily slop and that's what you give them. If it's vitamins you want them to have, then GIVE THEM VITAMINS. That saves all that work you're doing, plus is safer, no margin for error like with this faddish method.
A breeder I know does not free feed. He says birds eat too much anyway, that people overfeed them. He feeds the proper amount per bird daily but that's what they get. So, he isn't wasting food and he sure isn't fermenting it.
The better choice would be to grow some good forage for them in a penned off area, then let them in there to eat it down and do it again. Fresh food, they pick what they want and what they know is good for them. What they're doing is an extension of babying their birds, IMO, but it's not doing what they think it is. And they think I'm fussy about my birds, sheesh.