We do know from TNG that Romulan ships use artificial black holes to power warp drive so I suppose it's possible that Scotty would not have been able to determine the vessel's wider capabilities but from a story perspective it makes little sense. If they believed that they had destroyed all the local bases they were intending to destroy, they could have warped away and there would have been nobody around to see. In season 3 the cloaks functioned at warp but these were an upgrade, admittedly.Which would fit perfectly with the WW2 U-boat parallel that inspired the episode: Kind of slow diesel engines on the surface, really slow battery-electric drive submerged.
Their warp trail would have decayed long before anyone could track them though, or so they should have believed.
Tactically, if they wanted to leave no recordable trace of their visit, staying cloaked and sub-light until they were out of the Neutral Zone would make sense I suppose but then they screwed that up by not encrypting or firewalling their communications.