You were on the mark until the last sentence. Grand narratives are an indispensable part of science. When we discover the right narrative (evolution, the big bang, etc) it helps to frame all of our subsequent thinking. His arguments are bad, but their attachment to a grand narrative are irrelevant to their being bad.
Grand narratives are different from epistemes. Grand narratives have assumed notions of absolute truth because of the power dynamic implied in those notions. See that second link. It's what postmodernism is all about. The tl;dr is epistemes = what knowledge is at a given time & grand narratives = who gets funded.
Jared Diamond tried to reclaim environmental determinism from its unsavoury origins, but he didn't realise that you couldn't repair something when the foundations were rotten.
