I'm not recommending it, but the more knowledge we have about how things work good or bad the better.
In my opinion it can work at what it sets out to do for some people. I think you agree we shouldn't make blanket statements about what works for people.
Anyway, I'm not recommending it to anyone, but in practise although they allow whole grains, the diet is mostly based around rice and vegetables and is low protein. This may be shit in all kinds of ways, but I think it can make people more flexible in some cases. I hear what you're saying that something similar didn't work for you. It may depend which factor in flexibility is predominant.
Of course for us switching to this diet to improve flexibility if it worked would be ridiculous, like switching to a mars bar diet to improve sprinting. It might work, but it will trash the body.