It is the greatest find of the Dead Sea scrolls, and perhaps the most important archaeological find in history, a complete copy of Isaiah dating back before Jesus. It is also the most quoted scripture among the New Testament books. Why doubt that he existed, unless you’re just wanting to be difficult.
It's always helpful and informative to find extra-biblical support for the biblical historical narrative. I'm sure that if proper investigation of the Temple Mount was allowed we'd see remains of the First Temple.