T'Pau could have simply assumed that Spock would have mentioned this detail to Kirk and McCoy (considering that they're supposed to be his best friends).
That would be a reasonable assumption in the absence of any other evidence. But when T'Pring challenges, McCoy says, "Ma'am, I don't understand. Are you trying to say that she rejected him? That she doesn't want him?" So it should be clear to T'Pau from McCoy's question that Kirk and McCoy have never even HEARD OF the challenge, much less know anything about what the subsequent fight will be like.
Given that challenging turns the woman into PROPERTY, I can well imagine that challenging doesn't happen too often.