According to Enterprise, the Vulcans who opposed Surak were known as "those who marched beneath the Raptor's wings." The implication being they do become Romulans. Also, Voyager's Death Wish has Q accusing the renegade Q known as Quinn of starting the 100 year war that resulted in the split between Vulcans and Romulans in one of his failed suicide attempts.
The idea of the Romulans being the descendants of Surak's opponents is by no means fanon at all. Maybe it's missing that One Direct Reference, but there's no real reason to assume it isn't so either.