Mission: Impossible Fallout plot inconsistency

When Walker reveals himself to who he thinks is Solomon Lane, he ends up getting caught by Ethan and the gang. He seems surprised and shocked and still tries to convince them he's with the CIA. However, when the CIA close in on them, Ethan says that the enemies are surrounded and Walker says something to the effect that they aren't. Then it's revealed that he has allies in the CIA squad there who shoot all the real CIA agents. How did he know that if he didn't know he'd be caught?