Except some how they got to there destination? Ok maybe they had a mothership somewhere. But its pretty clear they didnt travel there from Romulus on there own at only sublight speeds as a ship that would take dozens of years to get from one system to the next is stragicaly useless against a Federation fleet of warp capable ships.
The ship never tackled any Federation fleets, and we never got any evidence that it would have been supposed to. It existed to tackle Federation fortresses
, and it tackled those very well.
Within the confines of the episode, we got no reason to think that the trip from the RNZ to Romulus would be interstellar. Quite to the contrary, the trip involved passing by a comet that had a tail, suggesting that all the action was taking place deep insystem (supposedly in the system that held planets Romulus and Remus).
It is only in TNG "The Enemy" that we first truly learn that the scale of things might be different. There, it takes a modern, definitely high-warp-capable Romulan warbird at least an hour to cross from the inner surface of the RNZ to the outer...