They have to brush im aside because what he says doesn't serve their intentions.
BTW. The same critics accept what they Muslims say about Mohammed, although there is no proof that actually said what is in the Koran. The oldest manuscripts date more than a hundred years after his death.
The same critics accept what the Muslims say about Mohammed Well, there is a pretty good description about how he came to start reciting the verses of the Koran. The messenger squashes him and forces him to start reciting.
I can accept that he might have been visited by an angel.
I just don't think he got that angel's identity correct.