The exact definition of how a nobody can be formed is that you have to lose your heart and it has to become a heartless, there is only one case where there is not a corresponding heartless to a nobody, and that is Naminé.
(However, it has been disputed over and over that Naminé isn't even a nobody, DiZ only called her that because he had no better term.)
When Master Xehanort extracted his own heart he let it not succumb to darkness but used one of the techniques only a true keyblade master can do.
His body remained there quite a while when it controlled his own heart so it's vastly different from what a nobody is, not to mention nobodies didn't even exist yet at that time because no heartless had access to the Realm of Light.
In fact, as mentioned by Yen Sid, it's actually pretty rare that when a heart is lost it leaves enough "impact" for a nobody to be born.
The unlocking of a heart does not produce a nobody, when that heart is afterwards taken by darkness it forms a nobody, which is not the case with MX here.
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So, that was the explanation by our current knowledge.
Yet still there remain some iffy things regarding another form of Xehanort, particularly the fact that Xehanort's body did remain there after the heart was extracted and was able to guide it into Terra's body.
Secondly there's a hint from Nomura that the "fixed sentiment that remained after MX stabbed himself" is not a heartless.
That leaves 2 options:
1. It is really another nobody, then the definition on how a nobody can be born has to be re-difined or
2. It is a completely new type of being we don't know yet.
Nomura also hinted somewhere that we wouldn't be able to explain/understand the Mysterious Figure with our current level of knowledge, which would support claim 2 in the event that Xehanort has another form of a being that is neither full being, heartless or nobody.
On this one should also note the fight with the Lingering Sentiment in KH2 Final Mix...at the time it came out the LS was also not explainable with the current knowledge.
