Wouldn't be the first time something in the first season of TOS regarding warp drive or the transporters didn't jibe all that well. Like BoT and the question about whether or not Romulans having warp drive. Lot of debate on the meaning of the wording. Obviously they needed FTL capabilities but Scotty's words seems to contradict it. Then you get into the whole issue of if he's referring not to propulsion but power generation. Which is all well and good but the discussion in that scene seems to be a propulsion specific. Warp and impulse are means of propulsion in my book, not means for power generation. Fusion reactors, matter anti-matter reactors, or quantum singularity reactors are power generation. It's like nuclear propulsion on ships. It's the propeller that drives the ship. The reactor just provides the power. Or you get into the idea that maybe the Romulans had some other means of FTL travel other than warp drive, which I won't argue against. Perfectly happy giving Romulans mass effect cores to fill the gap.
"Don't confuse facts with reality."
-Robert D. Ballard