According to Dax on DS9 there was a Cardassian exile living on Vulcan in the 22nd Century, so the species was known, but official first contact probably hadn't happened yet.
Also what Romulan retcon?
Well, we don't have any official First Contact date with the Cardassians, so they could be in contact with the Federation in the SNW timeline.
As to the Romulans, weren't the humans in Balance of Terror surprised that the Romulans were Vulcan-relatives, because up until then they never had visual contact with the Romulans, (similarly to the Tholians)?