That quote just screams MX.Xemnas said:So...Sleep has taken you once again.
So, why would he end up being a part of Xemnas? Let's think back to another quote:
MX said:You say that now, but your heart is destined to become only a part of mine.
You see, MX knew that Terra's/Xehanort's heart would submit back to the darkness, giving him the opportunity to take control of their heart and absorb it. Only this time, Xehanort would not resist the absorption like Terra and Riku did (assuming XH tried to absorb Riku; the latter at least managed to temporarily control the body just like Terra did). Xehanort would not fight the darkness of MX's heart, instead he would accept it.
Now comes the part that I can't even comprehend that well. Because MX would absorb Xehanort's heart at the same time that the latter becomes a nobody, Xemnas would inherit Terranort's and Xehanort's memories. I said "Terranort" because this is him:
Not this:
Thus, when you fuse Terranort's spiky hair and Xehanort's long hair:
You get this:
I'm working out the other details, such as XH's sentience, why MX wouldn't be in control of XH, Eraqus' role (though I think he's the reason why XH has sentience), and whose keyblade is involved when XH opens doors, but I figured I'd get some CnC first.