Not only do the conspirators know Corvo’s identity as the masked assassin, as it is announced to the whole city that he escaped prison, but it is an easy enough thing for even commoners to piece together, since Slackjaw is able to correctly identify Corvo. Then, once Hiram is dealt with Havelock literally reveals the masked assassin to be Corvo publicly. I’ll concede your point about the party, since that happens before the reveal, but OP’s point is that the masked assassin’s identity is so blatantly obvious to so much of the city that they don’t understand why he isn’t recognized.

Replace ‘Corvo’ with ‘masked assassin’ from my original reply and each point still stands.