Ever since the Demon Fleet Leviathan teleported into the seas of Pandapolis, Stalingrad had been tracking the movements of the ships and getting information via the Deep Ones. It all served to provide Lord Wolte and the cultivators with the bigger picture of what was actually happening in the fog, helping them make decisions.
It was easy enough to predict that Focalor's ship was going to retaliate, so Wolte had already started its engine and moved away straight after attacking.
Using the loud blast of his main cannons, Wolte had been able to mask the nuclear engine's ignition as he moved away from the spot. So by the time Focalor actually fired his cannons, it was already too late.