It is also quite possible that the Norkan Colony massacre (and
the infamous defeat at Cheron) took place mere months before "The Defector".
I mean, it would be quite difficult to get information on who did what in a conflict long ago, when Romulans had had fewer opportunities to share information with the Federation. But after "The Neutral Zone", the Romulans might have gone quite active and interactive in an all-new strategy: "TNZ" did mark their first open onscreen attempt at cooperation with Starfleet, although there was probably a lot of covert cooperation going on during ST6:TUC already... The identity of the man who massacred Norkan might well be "public", UFP-accessible knowledge under the new strategy.
A massacre or two between "TNZ" and "The Defector" would be quite in line with the overall Romulan policy of unapologetic aggression at the time. The Star Empire was quite ready to risk war with all sorts of schemes, several times including the destruction of the Federation Flagship. Indeed, it seemed to desire
war on several occasions, although perhaps only as part of a bluff of some sort. Jarok's own belief that another destructive war would be unwelcome is unlikely to have reflected the actual thinking of the Romulan leaders at the time.