There's more to it than length. Should they have the fairways of similar width, rough of similar height, greens of similar speed and firmness? For that matter, should they be trying to match the course they're playing on? By the logic some here have, they should be having the qualifier on courses that approximate Pinehurst as closely as possible. If they play at Pebble Beach, they should try to approximate that.
Hopefully you'll agree this is silly. I think worrying over having a qualifier shorter than the US Open length is equally silly, and just isn't that big of a deal. As Jeff said, the qualifiers aren't going to win anyway, so what difference does it make if the set of qualifiers are the ones who will play the best on the tournament course or not? But if you want to match the qualifier courses to the tournament course, you need to worry about much more than length. What if the qualifier is 6800 yards but wide open, and the tournament course is 6400 but super tight? A long wild hitter might clean up in the qualifier, and shoot a pair of 80s in the tournament.
Sounds to me like some people don't like the idea of an 11 year old playing, and they're looking for ways to keep her out without having to add an age requirement. What happens if next time the 11 year is 5'9" and drives it 260? Have to find another way to keep her out then, I guess!