R(DLA)/100 deals specifically with this issue, justabout.
The legislation very clearly states "...impairment of intelligence and social functioning" NOT "...impairment of intelligence or social functioning".
Now, what that means is that you have to be able to demonstrate that your child is impaired in both areas.
So, a child who is of very low intelligence, but is otherwise unimpaired, would not meet the test.
A child who is of normal intelligence in all other areas but has absolutely no social functioning would not meet the test.
HOWEVER a child who has above average 'notional intelligence', ie. they score highly on IQ tests, but has, for example, absolutely no sense of danger at the age of 9 years old (as was the case in R(DLA)/100) can be argued to have a "severe impairment of intelligence" on the basis of useful intelligence, ie. the intelligence which is needed to live life.
I think that the guidance does give creedence to this in DMG 61367, which states that the DM should look at other factors which indicate severe impairment of intelligence despite an IQ over 55, one of which "is a lack of understanding of everyday living." No sense of danger would be argued. Also, DMG 61370 "The
term "social functioning" is what that person is able to do with the intelligence they have."
So I think the DMG does have it covered, and it issn't incompatible with NAS.