It's pretty clear that you enjoy the concept behind 'Assignment: Earth'. If it was it's own show without any connection to Star Trek, I'd enjoy it too. But no matter how much you try to justify why things happen the way they do, or how 'philosophical' the reasons are, it doesn't change my perspective that the whole episode is just a hollow attempt at creating a back door pilot for another show that would ditch the Star Trek brand rather than trying to expand it.
I don't need to "change" your perspective, but you are performing a number of cartwheels trying to tear down a great TOS episode. Further, "ditching the Star Trek brand" says nothing; endless spin-off series worked independently of the tone or influence of the parent series. Examples: the sitcom
Silver Spoons (NBC. 1982-86) was connected to
Diff'rent Strokes (NBC/ABC, 1978-86) with the Arnold Jackson character appearing on
Silver Spoons, yet there was no creative or "universe" linking the series at all.
Lou Grant (CBS, 1977-82) was a direct spin-off from the sitcom
The Mary Tyler Moore Show (CBS, 1970-77), but
Lou Grant was not only a drama, but did not retain any of the approach in characterization established on the parent series, or direct links to the former's "world". To that end,
"Assignment: Earth" did not
need to drag the TOS element / feel / universe over to a spin-off series, as it was clearly designed to be its own product. Backdoor pilots, crossovers and spin-offs do not operate by some hammered-in-stone edict saying all guest appearances or spin-offs "must" share the traits and approach to the parent series.
It's an episode that is so lazily written and executed that even the 'remastered' version featured it's own lazy additions like having the Earth rotate in the wrong direction.
Bad EFX artists" of the 21st century making mistakes has zero bearing on the episode.
Don't forget the line that came before that. "No such vessel listed. Records of that period are fragmentary, however."
The point is that Spock was acknowledging that historical records were not complete (and not just about Khan as he specifically says
"period"--an era, not a single event)--just as that is the case in real world history, hence the neverending need to study and uncover, thus justifying why Starfleet would authorize a trip to observe the serious problems of a years as disastrous as 1968.