Agreed, that was what is in the history books. And there's absolutely no onscreen evidence that Spock's briefing is not 100% accurate. But "canon" should not be a straightjacket; it's there to serve the story, not restrict the storyteller. There are lots of things that "appeared onscreen" somewhere or somewhen that I wouldn't hesitate to disregard because it would be so trivial to have inserted some other bit of dialog in place of that one. "Yeah, those Romulans were sure primitive back then; they didn't have phase guns or photonic torpedoes, or even Warp 5 drives. And they were so xenophobic that they didn't even try to communicate with us visually." That's just explication to set up the current situation. Everybody pretty much agrees that they had to have SOME kind of FTL drive to conduct an interstellar war, right? So the line about "simple impulse drive" has to be handwaved somehow. It would be more reasonable to fix that by ignoring the use of "impulse" there, than by concocting some technobabble about how "impulse power" can somehow be used to travel between solar systems in a matter of days or weeks instead of years or decades.
"Therefore, no human, Romulan, or ally has ever seen the other." That may be what the history books say; that may be what Spock believes; that may be the actual situation. But is it something that the modern audience would believe? I certainly find it hard to believe that T'Pau didn't know the truth, and maybe Sarek as well. I was half-kidding about Spock thinking that the Romulan commander looked like his dad. Canon would say that the two had an amazing resemblance; it would be illogical for Spock not to wonder if his dad had somehow gone over to the Romulans. (Joke!) (Well, maybe...)