[It's not a deliberate backup plan, at the very least. True, Xemnas isn't entirely ruling out the possibility that something will eventually force his hand one way or the other. But for now, at least, he's willing enough to simply see what this world might have in store. More notably, while he doesn't actually confirm Ienzo's comment about being trapped, he also doesn't deny it either.]

I could ask the same of you. However, if you truly must know, the last I recall is the events at the Keyblade Graveyard.

[How much of said events is... unlikely going to be something he cares to mention. But it does offer a general timeframe at least.]