They're operating in combat rounds. Xykon just completed his turn during which he cast meteor swarm and began to give chase. The last few panels are O-Chul and Varsuvius's turns. They've each taken a move action to reach the opening and are currently using free actions to speak (at length) about the situation. They still have standard actions left and then it's the familiar's turn; all before Xykon is free to act again...
...unless he's got an immediate action spell prepared.
Xykon is a sorceror; he doesn't prepare spells. Either he knows it or he doesn't, and he's never shown any indication of being able to cast more than one spell per round.
