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When has he not?
During KHII development, if he ever said to anyone that he was cooking up a great game, then thats one example.
He has a lot of the things planned, but definetly not all of it. Things like Xion's influence with Roxas was planned even during KH2's development, although Xion's character herself wasn't planned fully until they started Days.
It's a mix.
Nomura, so far, pretty much has kept his word for the most part. Some people just try to bash him because he didn't snap his fingers one day and have the entirety of the KH plot and details figured out over the duration of a few seconds. Others also try to bash him on games that haven't even been released yet, saying how "this shouldn't be possible", but it's best to ignore those who make such rash comments.