Complex sentence: Intellectual ability is measured as it has been found to moderate scores on tasks cognitive tasks presented in screeners and so may produce false positives or negatives (i.e., an intelligent dyslexic will use compensatory mechanisms, thus making it harder to diagnose).
Split into simpler sentences: Intellectual ability is measured as it has been found to moderate scores of cognitive tasks typically presented in screeners. This means that intellectual ability may produce false positives or false negatives in screeners (i.e., an intelligent dyslexic will use compensatory mechanisms, thus making it harder to diagnose).