I don't think they closed their borders. They invoked the Schengen protocol for re-instituting border controls. Under Schengen driving from France to Italy or Germany is like driving from Texas to Louisiana. But there is a clause allowing re-instituting border controls under certain conditions, including a national security emergency.

[edit: source France24; http://mashable.com/2015/11/13/france-border-airlines-flight... . Hollande did refer to 'fermeture des frontières' which is confusing https://twitter.com/Elysee/status/665314066106159104 ]