As I recall, Walter Koenig's explanation for that was that off-camera, Khan had to use the bathroom on the Enterprise for a good long time while Chekov was in there, and that's why he never forgot his face. That's my headcanon.

Anyway, the article says they're ignoring the blood that can reverse death, it seems, but the movie will still be in continuity. I can't say I blame them much because, really, immortality blood is an option worthy of its own film. Maybe in the future.