“We will massively retaliate” is one of the better deterrent postures, provided you can credibly claim that. But it might not be the best response if you’re actually attacked, because of all the horrific things a full-scale war would entail.

Iran had a front-row seat to the invasion of Iraq and a bunch of their leadership has personal memories of the Iran-Iraq war. They are probably trying to avoid a full-scale war if at all possible, and that’s probably what’s best for the Iranian people.