Several points:
For many of the things he writes about, we can take his clout and background as evidence that his insight is 'real'. It doesn't have to explained via careful science to be true. I think the fact that Paul Graham is saying something, for certain subjects, makes it highly likely to be very-mostly-correct. I'll happily believe it to a high degree unless I have reason not to. I believe the evidence that they're mostly-correct is that he wrote them, and the evidence that things he writes are mostly-correct is that they have been so in the past and that he's intelligent and moreover consistently intelligent, so we have little expectation of a given essay suddenly floundering into bullshit.
His insight and way of thinking can be useful even if they are unreal. I mean - I'm sure every essay on that list makes points that can be argued to death or outright refuted by a sufficiently committed pedant. But they still have value. We think in heuristics anyway, for the most part, so it's valuable to glean heuristics from smart people and to see how they think and how their heuristics work (or don't work).
Empirical evidence would not really improve many of these essays. For one thing, filling an essay with detailed evidence that isn't necessary for the reader to believe it would probably detract from the quality of the essay. And many of his points are opinions or perspectives. They shouldn't or can't be highly factual. They would become false if they were made hard-and-fast.
For many of the things he writes about, we can take his clout and background as evidence that his insight is 'real'.
But that's at most very weak evidence that his insight is real. Much of success is luck and networking, for example, and successful people are notoriously bad at disentangling various causes of their success properly, in favor of giving themselves more of the credit than warranted.
This isn't really a full post, but merely a note of potential interest. Paul Graham (who runs Hacker News) has several very interesting and thought-provoking essays located on his personal website. To me they fit very well with the style of thinking employed and advocated by many people on LW and I'd advise that nearly anyone interested in LW check out his work.
I especially recommend Keep Your Identity Small, What You Can't Say, and What You'll Wish You'd Known, but nearly every essay up there is interesting to me in some way. Many of them are directly relevant to issues of rationality, while others are only indirectly related, but either way I found them worth my time.