If they were screening based on academic achievement, it would make it more plausible that these were hot-housed or otherwise educationally-advantaged children, but since they are using the NNAT, which is more of an ability measure, I would think it is less likely that the early educational experience is the cause, and more likely that the early educational experience is an effect of coming from high-potential parents.

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