Very nice article.
Here are a couple for follow up...
http://www.metzerfarms.com/AngelWing.cfm?CustID=23452472
That one indicates too much feed (energy) in general.
http://www.metzerfarms.com/NutritionalRequirements.cfm?affiliate=undef&CustID=23452472
I personally consider it at least partly genetic... otherwise why does the rest of the flock being fed exactly the same not experience it? Some individuals may absorb or process things more or less efficiently than others.
I fed my ducklings and ducks Purina flock raiser (20% protein), in addition to daily free range and never experienced angel wing YET. I don't do treats.
Out of curiosity... what are you feeding including treats and supplements?
Hopefully 1 week isn't to long for correction... I would try as suggested in the article you posted.