What if the ship and mines in "Minefield" didn't use cloaking devices but rather used holographic camouflage in combination with stealth technology and sensor jammers to mimic the effects of cloaking and this wasn't discovered until the Earth-Romulan War? Do you think this theory suitably explains why the Romulans appeared to have cloaking technology a century before they were supposed to have it?
A much less convoluted answer: They simply ignored Spock's lines about cloaking devices being an amazing theoretical technology in "Balance of Terror", as well as what was said about the level of technology used during the Romulan War.
Remember, for the spirit of Spock's comments to work (he didn't refer to the Romulans but the technology of invisibility in general), you also have to ignore or work around the cloaking Xyrillians and Suliban from Enterprise too. Looking at Trek's "big picture", cloaks very much weren't theoretical but something proven to work in the 2150's. Therefore, it's easiest just to go with the retcon.