It's almost like saying "90% of Americans have below median literacy skills." In that one it is obviously false, since we define median as being at 50-percentile.
But this is not a lot different. To me "6th grade reading level" should mean "the reading skills of a 50-percentile 6th grader." But then the statement can't be true, unless you accept that the typical 6th grader is just as good at reading as a typical adult.
So.... how do you define "6th-grade reading level"? It seems that you are left with defining it as what you might wish it were. Which is going to vary widely.