And nobody ever said they lacked warp drives in "Balance of Terror" as a species. Scotty may or may not have believed that this particular ship lacked warp drive, but there was absolutely no mention of the Romulans being ignorant of the secret of the warp drive.
Of course, just like the status of the Romulans can change from "enemy to be feared" to "forgotten enemy who has fallen behind times and no longer is a credible threat", the status of Romulan technology could change from "can warp and cloak" to "no longer knows how to build warp drives or cloaks". It just takes some heavy-duty believing to accept that this sort of amnesia could strike in just a single century. And there's no particular need for it, as Romulan warp drive in the 2150s is in no way contradicted by "Balance of Terror".
...Now, the idea that the Federation forgot that the Romulans already had invisibility down pat in the 2150s is a bit harder to cope with. But perhaps Romulans went from major 22nd century foe to pitiful 23rd century nuisance specifically because they forgot how to cloak?