I would simply imagine that the Romulan cloaking device in BoT represented a quantum leap forward in the sophistication of the technology versus previous versions from DSC and ENT which were more about basic visual/thermal/"radar" stealth using holo-projectors and such.More foolish than Kirk and Spock look for thinking cloaking devices were theoretical in "Balance of Terror", when they were widespread just 10 years before?
Like the difference between our stealth technology today and that of ENT is equivalent to the difference between the cloaking tech of ENT/DSC and that of the Romulans in TOS. Their's may involve gravity manipulation to bend light using their artificial quantum singularity cores, for instance. To Kirk and Spock, that might come off as completely new and unfamiliar technology if we're willing to be a little flexible with the dialogue.
With so much drama in the D-S-C it's kinda hard bein' King-D-aniel-B.They don't care. If they want Borg or Romulans, they'll do whatever they want and "future" generations be damned.