Possible, this is a hypothesis that will be researched. The amount of water vapor blasted into the atmosphere was significant, but was it enough to force suppression in the N. Atlantic basin? I'd buy into some influence support tanking ENSO for an abnormally strong third-year La Nina. The timing of a hyperactive EPAC in July was pretty wonky though. Really everything is a bit wack and needs looked into.
Yea, nobody knows for sure and its probably a combination of factors.