Spock heard him and did not correct him. Ergo, Vulcan was conquered.
Doesn't sound logical to me. The whole context of the scene is about Spock trying to motivate the good doctor to join him in his investigation while McCoy wants Spock to join him drinking or getting drunk.
Correcting Dr. McCoy will not serve Spock's goal but rather the opposite.
Interesting idea but that would make McCoy more knowledgeable about details of Vulcan history than it usually seems.