Spock actually advises Kirk to strike first in Balance of Terror based on the probability that the Romulans are an offshoot of the Vulcans from a more violent past.
Yeah, fanon or no, there are elements from TOS and ENT that strongly suggest that the fanon assumption about Surak and the Romulans is supposed to be part of the backstory. It all fits together nicely.
The Romulans objected to Surak's logic-fetish solution to what ailed Vulcan society, so they left. But they still had to come up with a solution of their own.