This is what I didn't understand about Star Trek 2. I think since Sulu eventually was a Captain by Star Trek 6, he should have been the one to be the first officer on the Reliant in Star Trek 2, and it would have made more sense that he would have been the one to remember Khan, since Sulu was actually on the show since the very beginning (although I don't remember if he was actually in the "Space Seed" epiosde).