Xehanort doesn't exist until the very end of BBS.
And Organization XIII wasn't "just beginning" until a year after BBS.
And, since that Wikipedia article gives no reference to where it was confirmed that he was playing Terra, it could be as much as guesswork. Meaning it isn't concrete, nor is it applicable.
I know that Xehanort didn't become ansem's apprentice until one year after birth by sleep but, where was he for an entire year. Also how would he end up on Hollow Bastion/Radiant Garden if the final battle happened on Sunset Horizens.