They were the same species on the same planet just a few thousand years ago, which is the blink of an eye in evolutionary terms.
It might be more accurate to say that Vulcans are just Romulans with impulse control. Before the split, Vulcans were more like modern Romulans than modern Vulcans. Vulcans at that time were highly emotional and violent. Then they had a sort of cultural revolution, which involved controlling emotions and focusing on logic. This led to some traditionalists leaving Vulcan and founding Romulus, carrying that emotional and violent culture with them.
It's curious that despite this cultural revolution that led to founding of planet Vulcan they are still basically equals technologically. It doesn't appear that a few thousand years of excess emotion and violent tendencies has been at all detrimental to Romulan technological advances.
Their physiology is barely diverged so their intellects are likely to remain similar. Espionage is frequently the theme of Romulan encounters, which would help keep them up to date. And if they procreate more frequently than every seven years, they might have a much larger population even with greater murderousness, with more people being advantageous for tech development.
They also have an empire with a large number of client species. Vulcan does not. If anything the romulans are more advanced than the Vulcans
It’s kinda odd in retrospect. There are many words to describe Romulans… but violent isn’t really amongst the top ten.
I think the police state was their solution to infighting in much the same way that Surak's faction took up a quasi-religious adherence to logic.
