Again a happy camper,
CorporalCaptain is.
TWOK is nominally a sequel to "Space Seed," of course, but the continuity specifics are irrelevant for at least three reasons.
1. TWOK tells you everything you need to know about what went on in "Space Seed." Khan recounts the crucial events, such as they are, even characterizing them as a tale that Chekov could have amused his captain with. If that's not making the film accessible to anyone, whether they saw "Space Seed" or not, I don't know what is.
2. The specifics of what went on in "Space Seed" are irrelevant to the primary themes of TWOK, anyway. This fact is lamp-shaded in the middle of the film by
this exchange:
CAROL: I don't understand. Who's responsible for all this? ...Who is Khan?
KIRK: Well, it's a long story.
DAVID: We appear to have plenty of time.
KIRK: Is there anything to eat? I don't know about anyone else, but I'm starved.
The question is raised, and Kirk doesn't even bother to answer, because it's irrelevant. Khan is a one-dimensional character who has only one function: to bring about jeopardy that Kirk can't get out of and Spock must sacrifice to overcome. Everything else is window dressing.
3. The specifics of "Space Seed" are so irrelevant that they are quite mutable by TWOK itself. a) Chekov can be inserted into the background of season one TOS, even though he is nowhere to be seen, and prior to TWOK it was both a common and natural assumption that he wasn't assigned to the
Enterprise until later. b) The ethnic makeup of the supermen seems to change without explanation between "Space Seed" and TWOK. c) The amount of time between "Space Seed" and TWOK is at least a little on the fuzzy side. Etc., etc. All. Irrelevant.
Continuity is overrated, especially strict continuity, and canon, shmanon, that doesn't stop anything with higher purpose from happening.