The fact that Robin doesn't move a muscle is probably what saves him, ultimately. Irahl's instincts of Flight and Fight are both currently cranked up in high-octane overdrive, but Robin doesn't move in a way that tags himself as either predator or prey in that instant. So, that only leaves Freeze.

It buys Robin the second needed for even a single coherent thought to enter Irahl's mind, for the first time since he'd woken up in the bottom of this hole. He's still just as sure as he was a moment ago that Robin is here to drag him back into a cage, but he hasn't done so yet, which might mean nothing in the long-run, but Irahl is lost here and Robin is the only glimmer of familiarity he has found. He could shoot that glimmer in the head, but then he would be left with zero options and a much bigger mess.

So, if Robin needs one more jolt of reality to confirm the specter that he's seeing, he hears what is unmistakably Irahl's voice and all of the absolutely dire coldness that it is capable of holding.

"I'm leaving."

It is stated as unwavering fact. His tone leaves no room for negotiation. Robin has one chance to either help or get out of the way, because Irahl is heading up, whether with, without, or through him.