I think it is important to note that Arabs does not equal muslim. And I don’t know exactly the lineage that gets traced from Ishamel to the modern Arabs whether it is in the Bible or what. Just because Mohammod said he was descended from Ishmael doesn’t make it true, and doesn’t make his cult “true”.
:-)
And your lack of knowledge doesn't make it false, either.
The Table of Nations in Genesis 10 gives ample reason to believe, generally speaking, the peoples of the Arabian Peninsula are indeed descended from Ismael.
As for the validity of his teaching; well that part of the world has never lacked for false prophets, which in turn says nothing to impeach the validity of true prophets.