He's absolutely not saying iron caused the collapse, only that it prevented the recovery.

> The causes of the Bronze Age collapse are still debated by historians: a series of severe droughts; the sudden appearance of mysterious Sea Peoples, who raided along the Mediterranean coast; the disruption of the tin trade routes; internal revolts; or possibly all of these at once. But there is another unifying factor that could have prevented the resurgence of empires: iron.