I was looking up theories about that and I found this one that I totally buy into (well, not everything he says, but I like the direction this is going):"The truth is what you see with your eyes, not what you hear." Cue flash of light and total silence when MoM tells Xehanort his name. Suddenly the MoM is a personification of Kingdom Hearts idea seems more likely each time I come back to that scene XD
Either that or his true name is Fade-in Sepia Tone.
That would explain Young Xehanort's dumbfounded expression. (+ the goat on No Name which isn't mentioned in the video, but discussed in the comments)