String theory is a field of mathematics whose axioms are only intelligible to Edward Witten. Besides, it has produced a tremendous amount of conjectures and theorems that mathematicians would have been unable to come up with otherwise, so we can safely ignore Feynman's judgement.
I remember in the 1990s watching other physicists practically worship string theorists. Why? Because it was difficult? Weird?
I thought it was a fun idea, but much more like philosophy than actual physics. As Feynman says in the opening page of his Feynman Lectures, "The test of all knowledge is experiment. Experiment is the sole judge of scientific 'truth.'"
String theory is famously untestable, which is why physicist Brian Greene made a whole TV series asking the question, is it science?