It's a statement communicating her strength of opinion, I find it hard to understand how you perceive that as "fanatical" - what would a "scientific method of evidential support" even look like in a domain like business? Is she meant to publish a paper with p-values of his effectiveness running YC or being a YC partner? If she pointed to the success of YC during his tenure (which she could well have done with justification), you'd probably complain there were many other factors (true) and so was only association, not causation?