we must remember that Romulus and Remus are the Earth names for Romulans\Remans. Romulans and Remans, don't call themselves that in their own language.
ENT "Minefield" casts that very much in doubt.
When Romulans communicate with Hoshi Sato in their own language, she hears them identify themselves as the Rumalin. T'Pol corrects that to "Romulan" - probably because that's the preferred Vulcan pronunciation, as Hoshi must have gotten the Romulan pronunciation right on the first take.
Once it becomes known that these folks are the Romulans, the rest then probably follows. Earthlings learn (falsely!) that Romulans live on two homeworlds - so those become Romulus and Remus. Earthlings also learn of a system of representation, and apply whimsical Roman names to the various titles in that system. Likewise, they apply whimsical Roman names to the military titles and practices. All of this silliness is all the more to be expected since Earth and Romulus are at war at the time this all happens: the enemy is naturally cast in a silly light.
The Universal Translator at this time apparently isn't quite as advanced as it is in TOS and later Trek periods, but it may be blamed for some of these translations as well. Eager to satisfy its users, it may start to follow the "Roman pattern"...
...After all, it gave Kirk and Spock "troglytes" for a cave-dwelling culture, and it took some time for the heroes to figure this out. They
hadn't put the thought in the UT's cyber-mind. And apparently nobody else had, either, because Spock would have known what those putative other Feds would have known about the state of affairs at Ardana.