Either they made assumptions that don't hold, or graphene actually isn't "stable" as such but is stable enough in reality. I don't know much of the history of quantum chemistry simulations of graphene, but I'm fairly sure they existed beforehand and the experiments were a nice confirmation. Quite often it's possible to simulate things which are thermodynamically impractical, with things like an incomplete thermodynamic sampling in MD, or small periodic cell sizes artificially stabilising systems.
Either they made assumptions that don't hold, or graphene actually isn't "stable" as such but is stable enough in reality. I don't know much of the history of quantum chemistry simulations of graphene, but I'm fairly sure they existed beforehand and the experiments were a nice confirmation. Quite often it's possible to simulate things which are thermodynamically impractical, with things like an incomplete thermodynamic sampling in MD, or small periodic cell sizes artificially stabilising systems.