I think the author nails the key flaw for me - what experiences are you going to miss out on by living so frugally? Would I have gone travelling round the world had I'd been worried about saving 70% of my income? Or risked it on a new startup idea? Probably not. And by retiring so early surely you're condemning yourself to a life of careful spending so you don't run out of cash? No thanks.
It's worth avoiding unnecessary indulgence and everyone should probably save more but I personally feel a better strategy is to find a happy work/life balance and try and set some passive income streams up so you can at least relax earlier if not entirely retire.