That's not entirely true for those of us that are willing to take what Scotty and Spock said in context of TOS itself.
There's no way that Romulan BOP could have gotten to the Neutral Zone in a feasible amount of time if it only had Impulse Engines.
Scotty was guessing at that moment as to what the ship had, based on the minimal scans they were able to get through the cloak and his years of engineering experience.
Scotty was really good, but he wasn't perfect or infallible.
As this excellent YouTube analysis of the episode indicates, what Scotty meant was 'only impulse WHILE cloaked.' Between the cloaking device and the plasma torpedo launcher, the BOP only has so much energy to go around; it can't run the cloak and also run navigational deflectors and warp drive. It's hide or run. Those outposts are literally on the edge of the NZ itself, and at full impulse one can presume the BOP is just under the speed of light. It doesn't have to make it all the way into Romulan space itself - just into the NZ.