Spock not knowing anything about the Romulans right off hand could be because the Vulcans have been interstellar travelers for thousands of years.
Assuming Earth and Vulcan are not that far away from each other in interstellar terms, we'd have to wonder then, why they didn't colonize Earth during their "aggressive" period (i.e. pre-Surak)
The Romulans departure might not have been considered anything special at the time and no important historical note was made of it.
Though I would find it somewhat strange that such
a departure would not be recorded, my main issue is that the Romulans inhabit what seems to be a very remote sector of the galaxy (it even takes a subspace message considerable time to get from there to Starfleet) for which they probably needed warp-capable spaceships to get there in the first place.
But when the Enterprise
encounters their "flagship" it only has "impulse power". This always stroke me as Vulcan offshoot that had to reclimb the technological ladder to re-acquire these skills.
There are different and interesting points of view how to interpret the "impulse power" of the Romulan flagship and the meaning of "Romulan Star Empire" in the Tech thread about the Romulan Bird of Prey
(from "Balance of Terror").