I'm not entirely sure if I believe the X-Antibody is that ancient of a code. From what I read about it, it altered the appearances of some digimon, such as Agumon, Gabumon, Greymon, and Garurumon. Since Adventure has the original forms of these digimon, I feel it's unlikely that the X-Antibody had been introduced yet.
I have also found nothing on Digimon X-Evolution taking place before Digimon Adventure. And it wouldn't make much sense for it to take place before Adventure for the very reason I highlighted.
Yggrasil (sp?) wanted to create a new digital world. He destroyed the old one and made a new world for digimon to live on. Most of the digimon were wiped out (because Yggrasil wanted them gone) and the ones that did survive were being hunted down by the X Program. Those with the X Antibody survived, but it also changed their appearances.
For instance, here's the X forms for Agumon, Gabumon, and Gomamon:
These digimon do not appear in Adventure looking like this. So the conclusion I can come up with is that the events of X-Evolution had not happened yet, or took place in its own timeline. It's very possible that X-Evolution takesp lace in its own universe. Look at the seasons of digimon. Except for the first two, the rest are standalone universes. Oh and to point out, the whole "Digimon Adventure is a TV show in Digimon Tamers" is actually a dub change. In the original, there is absolutely no talk of a TV show at all. No talk of Tai or the other DigiDestined.
Is there any data that confirms X-Evolution taking place before Adventure? Any official japanese sources that, in translation, confirm this? I have yet to find any such sources.
