The thing I am getting at is that "retcon" means "retroactive continuity." It doesn't matter what Khan says during TWOK about what happened in "Space Seed" -- if we didn't see it on screen and none of the characters say or otherwise indicate that it happened -- it didn't happen. Hence, retcon.
I know what a retcon is.
Your interpretation of retcon is more severe than we are used to seeing and using. For me, a retcon infers that we are supposed to forget a previous canonical factoid because it's just been overridden by a new canonical factoid. But TOS never said that Chekov wasn't on the ship during "Space Seed", so ST II didn't overwrite such a statement.
That made me laugh out loud. Thank you.
I just rewatched Space Seed and The Wrath of Khan back-to-back. In one scene in the movie, Khan quotes a Klingon about, I think, revenge being a dish best served cold. How is that Khan knows a Klingon proverb? Didn't he leave Earth in the mid/late 1990s cryogenically frozen, then wake up 200 years later on Kirk's Enterprise (in Space Seed), and get exiled on a planet until the movie?
I don't think he had any computers on the wasteland planet Ceti Alpha V became after VI exploded. Unless in Space Seed when Khan had access to Enterprise's computers he had to stumble on Klingon proverbs as he was studying the starship's schematics? 