I think that Epsilon IX was referring to in-universe. And I agree that they could have existed alongside with the ship classes of the NX-Project. Perhaps ships of the UESPA were used mostly for defense and exploration prior to the Romulan War, and were used to fill the ranks, while ships developed as offshoots of the NX-Project were used as command ships, or stayed behind in friendly territory for defense. The differences in design could be explained by different manufacturers.
I think it is very much a shame that the executives weren't willing to make references to Masao's designs. I don't think they would have found it much trouble to ask for permission. I think they should have also made a poll so they could find out how much we would be accepting of deviations from what is typical in design for Star Trek vessels. I think that they should have done the opposite of how Gene Roddenberry told Matt Jefferies to design the Enterprise, and instead have her designed based on how it was expected that aerospace design would evolve over the next century and a half.
The more that I look at Masao's designs for early Romulan craft, the more that I appreciate how alien that they look, though I'm kinda
when it comes to their weaponry. I mean, not to an extreme, but I'm thinking how inconvenient it might be that all of it is fixed forward.
OTOH, this WAS started with the original Romulan Warbird, and the same thing was done with the ENT-era Warbird. I'm just saying I like how most Earth vessels from that era have turrets and a flexible weapons coverage, with few exceptions (I'm looking at the Torsk in particular; fixed weapons are okay for fighters like the Minotaur, but Cruisers?), and I think that I would have given the Romulan vessels turreted weaponry as well.
Though that is not to say that I'm not open to the idea that Romulan ships have such great maneuverability that they can bring their weapons to bear while skirting aside enemy fire; they have been in space for thousands of years, so it is reasonable to assume that their technology is some of the most advanced in this part of the Milk Way.