They note 2006 as a "notably active exception" but that is almost universally reviled as one of the worst overall chase seasons since the turn of the millennium. Most of that activity was in the jungles, and it shut down after April 15. May was dominated by a cool eastern trough. Based solely on my arm chair knowledge of meteorology/climatology, ENSO is overrated as a predictor of US tornado activity and chase season quality. PDO/PNA seem to play a far bigger role, although the details as to how and why are beyond the scope of my knowledge.