I examined the context of aforementioned events as part of my Enterprise-C thread.
PICARD: Enterprise C? She was lost at the battle of Narendra III, defending a Klingon outpost from the Romulans.
GUINAN: And the survivors?
PICARD: There were stories of prisoners taken back to Romulus, but these were only rumors.
("Redemption, II")
I think it stands to reason that there had been Klingon survivors from Narendra III (possibly the re-appearance of the Enterprise-C bought one or some enough time to evacuate) to live and to tell the story.
The rumors were probably spread by merchantmen that smuggled Romulan Ale out of the Neutral Zone (and possibly Tasha tried to escape on one of these vessels with Sela).
There had been no communication whatsoever between the Federation and the Romulans since the Tomed Incident, 30 years before and 19 years after Narendra III.
Other than that I have absolutely no doubt that the Klingon outpost on Narendra III was destroyed either way and in both timelines / universes.
"Battle-weary" Picard in "Yesterday's Enterprise" only speculated when he said "The Narendra Three outpost was destroyed. It is regrettable that you did not succeed. A Federation starship rescuing a Klingon outpost might have averted twenty years of war."
The Enterprise-C had suffered critical damage and apparently had not taken out one of the four Romulan warbirds, yet. The "last ship standing" must have been at least one of these warbirds (in order to take Tasha and the other survivors as prisoners to Romulus) and I'm sure the Romulans finished what they had begun (trying to frame the Federation for the assault, why else should the Klingons have turned onto the Federation and not the Romulans?)
I think the "universe at war" Riker, unlike Picard, understood the situation much better: "That won't accomplish anything, sir. There's no way they can save Narendra III."
Bob