I'm not here to defend the American education system. It's just a big jump from "this issue is so widespread only a failure of teaching can explain it all" to "learning disabilities don't exist".
Learning disabilities do exist. But wrt a 6th grade reading level, it would be shocking if learning disabilities were anything less than a severe minority of the cause. That’s not to say, however, that improper teaching in response to learning disabilities are necessarily a minority of the cause, just that the disabilities cannot possibly be so severe that we should accept this outcome.
I probably worded it very vaguely.