I'm not sure any degree of questioning would have revealed the identity of those attackers - something else, comparable to "Balance of Terror", must have happened in addition.
Supposedly, Romulan is almost identical to Vulcan, hence the need for Uhura's expertise. It's a bit dubious that any amount of audio contact with Romulans would amount to the identifying of exactly three ways in which the speaker can reveal him- or herself as Romulan... I mean, one could tell an East German speaker from a West German by dialect thanks to existing knowledge of pre-split German dialects, and one might spot an East German agent when he says "Elast" instead of "Plast". But if one seals a peace treaty with somebody whose language differs only ever-so-slightly from German, one would have to be remarkably stupid not to consider the possibility that the enemy is German!
"Balance of Terror" and STXI together must be taken to mean that the old peace treaty was negotiated in English, or that the Earth negotiators had a good reason to think that the opponents speaking odd Vulcan did not have it as their native language.