The Iranian missile arsenal and production is now focused on solid fueled missiles, so they were emerging from a predictable place, but with a fuelled rocket they could fire in minutes.
Since it took at least an hour for a cruise missile to make it's way and around 15 minutes for an ALBM to make it's way, that means that once Israeli drones started being shot down, they were able to dig out the entrances, exit and launch faster than it took for a munition to be delivered.
Of course if Israel had been able to fly manned aircraft deep into Iran and for prolonged periods of time, that would have been impossible, and they would actually have been unable to fire. But that wasn't the case and so they were able to fire, probably limited by the ability to dig out the entrances and synchronize launches between different sites.
Since it took at least an hour for a cruise missile to make it's way and around 15 minutes for an ALBM to make it's way, that means that once Israeli drones started being shot down, they were able to dig out the entrances, exit and launch faster than it took for a munition to be delivered.
Of course if Israel had been able to fly manned aircraft deep into Iran and for prolonged periods of time, that would have been impossible, and they would actually have been unable to fire. But that wasn't the case and so they were able to fire, probably limited by the ability to dig out the entrances and synchronize launches between different sites.