Yes, you raise a valid point, but it is not “this supudiety again.” Just because Jesus is not established as a historical figure, except perhaps by Flavius Josephus, does not mean that we should not be demanding of evidence about Muhammad.
Disclosure: I accept that Jesus existed in part because of how consistent the account of his life harmonizes with the writings of the Prophets, who were many people who wrote over a span of many centuries and whose works were preserved in an integrated collection.
Mohammad is as fictional as Santa Claus.