Since the term was never used in TOS to describe the Romulan ship ( only its markings) calling the Klingon ship in TSFS is not 100 percent wrong or inconsistant. The implication from Stiles comment is that every Romulan ship seen in the Romulan War had this marking. Its unlikely that all Romulan ships were of the same design. Its an "national identifier" like a red star on a Soviet aircraft. It reflects the ships origin not its type.
OTOH the type ship seen in TSFS may actually be called "Bird of Prey" in Klingonese as well as some other alpha-numeric designation. From a visual standpoint it looks more like a Klingon design anyway. Romulans might also have ships they call "Birds of Prey" in their language. Ditto for "Warbirds". Perhaps thats why such terms use qualifiers like "Romulan" or "Klingon" when mentioned in Federation Standard.