You're assuming that simply because the Romulans had no contact with the Federation, that they had no contact with other neighboring species.
The thing is, the episode itself is explicit about it: the RNZ completely isolates the Romulans from everybody.
Not everybody. The episode is also explicit about the fact that the Romulan Commander is weary of constant wars against . . . somebody.
The Romulans couldn't have been too
isolated if Mark Lenard has spent his entire career waging wars on behalf of the Empire . . . . .