@Christopher - Sorry, I got carried away and should have been more specifric - they were always intended to be the warp engines on the USS Enterprise, and were referred to as nacelles (Star Trek is remarkably consistent despite what some people say) - but I was trying to make the wider point that the similar structures on a Romulan Bird of Prey in that era might not necessarily have been conceptualized by staff as being there for the same reason - I do not know how "firm" the TOS staff's conception of a warp engine being present on all alien vessels was.
Actually, the intent behind the design of the Romulan BoP -- based, I think, on a line deleted from the script -- was that the Romulans had stolen/copied Starfleet technology through espionage, which was why the ship's saucer and nacelles looked so much like those of the
Enterprise.
As far as I am aware, and correct me if I am wrong here, the concept they they generated a field that warped Einsteinian space-time was not solid, or written in a series bible, as of TOS - there may have been speculation amongst advisors, fans and scientists in this direction, but as far as I remember the field-coil warp-plasma thing only got set in stone with TNG and the Tech Manual - so I genuinely don't know whether the Klingon D7 or Romulan Bird of Prey, which were the only significant alien starship models in the show intended them to represent alien forms of warp drive, as we now conceive it, or just indicate general parallels in technology level.
"Warp" means bending spacetime by definition. There's really no other way of defining it. They weren't pulling these terms out of thin air; Roddenberry drew on research and consultation with scientists and engineers, as well as decades of SF literary precedent. The term "warp" for FTL drive based on distorting spacetime dates back to the 1930s, although I believe Trek was the first to use the specific phrase "warp drive."
Anyway, all that stuff about field coils and such is splitting hairs. The topic on the table is whether the Romulan nacelles were intended to represent components of an FTL drive, and I think it would take an extremely labored convolution of logic to pretend they weren't. Okay, yeah, Scotty said "their power is simple impulse," but as I suggested above, that doesn't necessarily mean they
don't have warp drive too (it could be "simple impulse" as opposed to "advanced impulse" or something). And even if that was the intention of the script, the scriptwriter didn't design the ship. There are a couple of things in BoT where the scripted dialogue and the onscreen VFX don't quite line up -- for instance, the dialogue says the Neutral Zone surrounds the twin planets Romulus and Remus, suggesting it's a zone around a single star system, but the onscreen map shows a border between two interstellar territories and throws in the nonsense word "Romii" instead of using Remus. And of course there's the bit where the phasers are fired but look like torpedoes. So the script and the finished effects don't line up perfectly, because they were done by different people.
Well they did that with TMP, redesigned basically everything, iconic elements included, and everyone was cool with it. In three in universe years, almost every aspect of Starfleet aesthetic and technology changed, and that was just fine
Oh, not everyone was cool with it. There were some fans who refused to accept it as the same reality as TOS. There are
always fans who denounce every new incarnation of Trek as illegitimate and wrong, and they always make a huge amount of noise about how "true fans" will never accept the new abomination... and then 20 or 30 years later, everyone forgets they ever existed.