I don't see that even a real attack taking out every ATM in Iran would qualify as the largest anything in history. There aren't enough ATMs in the country.
The phrasing could be bad, given that it's a (likely) ESL source: they might mean it's one of the largest cyberattacks on Iran in history.
(I don't have a factual opinion on the claim, only that this phrasing interpretation seems possible to me.)