I can see Nomura right now going over the plot for KH3D and KH3. Nomura: *going over story details, notices his Xehanort mess* "lol Nope." *brings back old Xehanort*
If anything, Nomura was going over the whole thing thinking "how can I throw in
another twist in the whole issue...oh right, lets bring back Xehanort as MX and otherwise keep total radio silence about Terra and/or Eraqus. The fans will surely go nuts and theorize/interpret like no tomorrow."
Now look at what happens ever since DDD came out...
Not that I like taking Xehanort literally since he isn't the most honest character, but Young Xehanort already stated that the Master Xehanort we see in KH3D is the most current form of himself. If that were not true, Master Xehanort would not have been the last to appear and end the time travel loop (or whatever you want to call it) in their most "present" time.
The old man we see is the Xehanort that was brought back.
That is why one of the unanswered mysteries of KH3D is "What happened to Terra's heart and body?"
The Memorial Ultimania also confirms it in written form:
It explicitly says Master Xehanort's
true body was revived (meaning the old geezer one), so the first question
When Ansem SOD and Xemnas were defeated, was Xehanort recompleted as MX?
has to be answered with a clear and indisputable
yes as it is an established fact so far.
The second question is deliberately left unanswered to foster debate and theorizing and play up the "Terra/Eraqus/Xehanort"-mess even more as the unanswered mysteries quote from 3D shows.
Considering Nomura's need to make everything "surprising" and fill some plotlines with more twists just for the heck of it, I'd bet this is just another of those cases.
Nomura needs to learn that while sometimes answering a plottwist with another twist can be entertaining or even rewarding if done with caution, it has to be done sparingly and not by dropping a whole pileup of twists on every second plot point, lol.