Find out what "controlled crying" actually is before commenting. It's a bad name for it and the right way should be described as controlled "comforting". Don't knock something that you don't know about fully. The media love to put a spin on things -kids are not supposed to be left for 20 mins at a time etc.
Done correctly, it can have a very positive outcome for all. And mostly the bubs. But don't ever try before 6 months of age. It will have no effect other than stressing you and eveyone out.
Whether it's called controlled comforting or controlled crying, I think we're all pretty au fait with the method whether we agree with it or not - it involves ever longer periods of allowing a baby to cry, and to teach themselves to settle. I think most of us know about controlled crying (or comforting, if you prefer) from more than the one source cited here.

I'm interested in the "don't do this before 6 months" idea. We all know babies mature at different rates... What's the cognitive change at 6 months, particularly, that makes controlled crying/comforting acceptable from a developmental point of view?

You are also assuming some kind of standard that professionals use to define what is acceptable and what's not. Believe me, after having five children, I've heard every permutation of controlled crying (or comforting) come out of professionals' mouths, from letting a baby cry but never leaving its side (patting its back, etc), to letting it cry for as long as it takes. I've seen women berated in mother/baby units for not wanting their child to cry until it vomits all over its sheets, for up to half an hour or even more.