It's kinda hard to tell either way whether he "unretired" before the end of the original series or not. He at the very least took a break at the end of the series (where he spectated the Ceremonial Duel), but aside from likely the upcoming movie, we have zero indication of him ever dueling seriously again, since he never did so in GX either (though I'd say Pegasus's opinions of the "top duelists" in Aster's flashback point to the fact he probably did duel again at some point between the Ceremonial Duel and GX. Which I suppose is now going to be cannon, with the new movie. Heh.). It's pretty clear that by GX, his days of dueling competitively ended a while ago, lol.
You really can't talk about Dark Side of Dimensions and GX in the same continuity though. GX obviously followed along from the Yugioh DM anime, but Dark Side of Dimensions is a continuation of the Yugioh manga.
Yugioh manga =/= Yugioh DM (anime); they're different continuities.
Not to mention, the implications of the ending of Dark Side of Dimensions is Kaiba pretty much commits suicide by using his VR machine to reach and duel Atem in the afterlife <..<
Edited by MidnightDaybreak, 29 October 2016 - 09:25 AM.