You're overlooking the simplest solution. Magus simply remembered that they would be there. Remember, Janus confronts the party soon after they arrive in Zeal. Janus grows up to be Magus, and once Magus returns to Zeal, it wouldn't take to my time to get reoriented as to where he was in the timeline, based on his memories, and become a prophet. He even states that this was his strategy at North Cape.