The thing is, "Romulans as a culture don't know how to do warp" does not work at any level in "Balance of Terror". If they don't know how to do warp, they are utterly harmless as an enemy - it will take them centuries to reach the nearest human target. The correct response to the destruction of a handful of outposts is to shut down the remaining ones and let the helpless Romulans stew in their own juices.
Obviously, Romulans must know how to get from star to star in a short time, far shorter than is allowed by the TOS definition of impulse travel. So if Scotty is saying "their power is simple impulse", this either
a) doesn't mean they can only do impulse speeds, or
b) is false, as can be expected from a speculative analysis of a ship Scotty can't even see!