This article is a little dated - PG said much later that after observing hundreds of startups do YC, Lisp (and programming language choice in general) is a lot less important than he'd originally thought.
There's a kernel of a crucial idea in there - to quote Marc Andreessen, "Differentiated is better than good." Lisp forces different tradeoffs than using say C or Java would; some tasks that are nearly impossible in Java become trivial in Lisp, while some tasks that are trivial in Java are really difficult in Lisp. That forces you down a different route than your competitors, which makes you stand apart from them in your customers' minds. And being able to explain to your customers why they should use your product over everything else out there in the market is pretty much the definition of a competitive advantage.
There's a kernel of a crucial idea in there - to quote Marc Andreessen, "Differentiated is better than good." Lisp forces different tradeoffs than using say C or Java would; some tasks that are nearly impossible in Java become trivial in Lisp, while some tasks that are trivial in Java are really difficult in Lisp. That forces you down a different route than your competitors, which makes you stand apart from them in your customers' minds. And being able to explain to your customers why they should use your product over everything else out there in the market is pretty much the definition of a competitive advantage.