What did Willy Wonka mean when he said across the desert lies the pomised land?

It is a biblical reference. He means that what she seeks (her son) lies at the end of a journey (to the other side of the factory, and in this sense is somewhat an overkill usage of the metaphor). Wonka's reason for using that exact quote might be relatively meaningless beyond the obvious implication, or it might bean insulting jab. In the case of possibly being an insulting jab; Wonka might be referring to the woman's German heritage in which case he is rubbing salt in the wound of WWII Nazi shame whereas the German Nazis murdered millions of Jews. Using a Old Testament reference of a Jewish journey from The Bible would be ironic in this sense towards a German woman in, of all places, a candy factory.