And I fail to see how hundreds of Vulcans being killed would seem unimaginable to Vulcans because Vulcan has never been conquered. That seems like a non sequitur.
Looking at the dialogue, the connection between never being conquered is not made because of the number that died. It's made because the Vulcainians' logic couldn't permit them to believe they were being killed by something in the zone of darkness. More specifically Spock points out that the fact that the zone is a negative energy field was illogical and also that there was something else there generating that field.
The Intrepid's crew couldn't acknowledge that this zone(and the thing inside) was killing them, becasue their collective memory (of never having been conquered) was telling them that it was illogical that they were being killed by this thing.
It sounds kind of illogical to me, but that's what the dialogue says.
MCCOY: All I can contribute is the further we travel into this zone of darkness, the weaker our life functions become, and I have no idea why. I'm all right. It's those stimulants. They catch up with you.
KIRK: Engineering.
SCOTT: As to the power levels are concerned, everything is acting backwards, but the drain is continuing. On the static test, I could only get sixty percent power. We're still being dragged forward.
KIRK: Spock.
SPOCK: It's logical to assume that something within this zone absorbs all forms of energy whether mechanically or biologically produced. Whatever it is, it would seem to be the same thing which drew all the energy out of an entire solar system and the Intrepid.
KIRK: The same thing, not the zone itself?
SPOCK: I would say not, Captain. The analysis of the zone indicates it is a negative energy field, however illogical that may sound, but it is not the source of the power drain.
KIRK: Maybe it's a shield of some kind. Some form of protection for something else.
SCOTT: But what?
KIRK: We'll find out what it is, but we'd better get out of here ourselves. Scotty, channel all the impulse and warp power into one massive thrust forward. That might snap us out of the zone.
SCOTT: Aye, Captain, but I'll reserve some for the shields in case we don't get out.
SPOCK: I submit, Mister Scott, if we do not get out, the shields would be extraneous. It would only prolong our wait for death by a short period of time.
KIRK: Use whatever power is necessary to get us out of here, Scotty. Report to your stations, continue your research. Dismissed.
(The department heads leave Kirk and Spock alone.)
SPOCK: Captain, the Intrepid would have done all these things too, and yet they were destroyed.
KIRK: Well, they may not have done all of these things. You just pointed out how illogical this situation is.
SPOCK: True. It is also true they never knew what was killing them. Their logic would not have permitted them to believe they were being killed.
KIRK: Explain.
SPOCK: Vulcan has not been conquered within its collective memory. The memory goes back so far that no Vulcan can conceive of a conqueror. I knew the ship was lost because I sensed it.
KIRK: What was it you sensed?
SPOCK: The touch of death.
KIRK: And what do you think they felt?
SPOCK: Astonishment.
There's the ringship.
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#shamelessplug
#shamelessplug