Take another look at Nicklaus' quote. The 1% percent that you are referring to are the exact 1% percent that are moving on to the PGA Tour.
Just because junior golfers play in those match play events on their way to the PGA tour doesn't mean they are proficient or getting any real practice at match play versus European players who play many more match play events on their way to becoming pros.
I mean, someone has to win those American match play events. If mostly Americans are playing in them, and Americans suck at match play, an American is still fairly likely to win. It doesn't follow that he's good at match play just because he won some events.
Heck, look at Tiger. He won plenty of match play events before he turned pro, but even he has a pretty lackluster match play record as a pro (which looks even worse when compared to what he's done in stroke play)