I think you misinterpreted it. the Road WAS fake until Billy made it real. It was a con so Agatha could steal other witches’ magic and she intended to do it with Jen, Alice and Lilia too but Billy accidentally made it real. He had no control over it, he didn’t even know it was him until the very end ! And it was still here after they all came out and he sealed the door. It means Billy is powerful enough to create something that can help witches get their powers back.