He's not making any claims
He's rather questioning Nin's credibility when making such pronouncements.
Not that there's anything wrong with anyone's contemplating this kind of thing -- we all do that; the problem is rather with blind acceptance of such musings as something well-founded and universally valid. Now, if those were words of Thucydides or Churchill, that'd be different. But Nin? What did she know about the matters that we must take it seriously. Just because she'd scribbled down what no one else bothers to, doesn't mean it's true.