The battle over the existence of Israel takes many forms. It is military, it is economic, it is political, and it is historical. One of the more innovative approaches taken by the Palestinians is to claim that they are descended from the Jebusites, a tribe which inhabited the area before the Jewish conquest. Fortunately, this is a claim which is easily disproven: http://www.meforum.org/article/1713.
The bottom line, I guess, is that if you hate Israel, you will find any reason to deny its legitimacy.