Though it is supposed to measure something similar to matrix reasoning/FR the NNAT is generally considered a screening instrument. The difference in scores between it and the WISC-V wouldn't concern me that much per se (except as it affects access to services). It's also usually group-administered, often on the computer, which is not to the advantage of learners with ADHD (vs individually human-administered WISCs).

...pronounced like the long vowel and first letter of the alphabet...