Indeed, nobody in the episode suggests the Romulans would "lack warp drives", let alone "fail to possess the secret of warp drives". The only thing stated, with some conviction, is that the individual ship they are facing has "simple impulse power" that puts it at a speed disadvantage.
And even that boisterous statement may be in error. Scotty is out of his depth trying to judge this invisible alien ship by what she has
not done so far. Essentially, he's an idiot, guilty of a basic logical fallacy: not having seen a "complex warp" powerplant in action so far is no basis for denying its presence. But experts often are. And there's little point in calling him for his error when the Romulans do go to warp... Especially as they still obviously can't outrun our heroes.
Context is King ...
What Spock considers "primitive" from his perspective 100 years after the fact and what we consider "primitive" from ours 100 years before the fact, are obviously going to be completely different.
This. The 1960s or the 2000s don't feature in it at all. Trying to evoke them is what creates the perception of inconsistency.
We can't tell what is primitive and what is not out of a pool of technology in which
everything is futuristic to us. Spock may consider virtual reality controls hopelessly antiquated as a concept, say; even for Archer, those might be something from the dark ages. We have to trust our heroes on this, plain and simple. And Spock is specifically using a qualifier to denote the differences between "then" and "now", essentially telling us that "then" was the same as "now", only more primitive. This works fine with ENT as "then" and TOS as "now",
exactly because the two are the same: Archer's phasers, engines and sensors are the same, only more primitive.
The two issues here are different altogether:
1) Is invisibility really "theoretical" as late as the 2260s?
2) Could Archer's space navy really not take prisoners of war?
The first issue is merely a source of embarrassment for the writer of "BoT": no space hero worth the title should be unfamiliar with invisibility devices, no two ways about it. The second is a continuity issue for real: the writers of ENT failed to create circumstances that would make it
technologically impossible to take prisoners, Romulan or other.
Of course, ENT along with the rest of Trek offers plenty of reason for Romulan prisoners being difficult to take: the trigger-happiness with the self-destruct systems, the Romulan fondness for proxy fighting with drones and slave races and illusions, and even the underlying need for this secrecy. It's just that this isn't technological as such. But Spock says the prisoners were not taken because the fighting did not allow for mercy - and any number of fundamentally technological limitations could amount to this in ENT style combat. Indeed, the only prisoner-capturing device available to Archer's navy, the transporter, could be utterly negated the first moment the Romulans turn on their shields; Starfleet would probably need decades to come up with tactics that make transporters valid again. And all the better if they do, because then Spock's claim that the capturing troubles were in the past is true again.
Timo Saloniemi