A piano concert's quality depends much more on the pianist than the piano, but you'll notice the pros don't give concerts on uprights.
If you have a choice of several languages, it is, all other things being equal, a mistake to program in anything but the most powerful one.
The point is that he thought at the time, and still (probably) thinks, that Lisp is language superior in power. As far as I know, no other language has eclipsed it, so the answer hasn't changed.
At least, that's the argument that has compelled me to re-visit it (after learning some Scheme in undergrad).
For example, the fact that 'foo creates the symbol foo if it doesn't exist is undisciplined. That means if you try to reference something in a certain package, then find you didn't import that package yet, then try to import the package, it will fail. The reason it fails is because you've already created 'foo, so it can't import 'foo.
That's just one example. Lisp is the most powerful code abstraction, but it needs discipline to succeed.
Left as an exercise for the reader: Are there any languages that have become popular despite not having discipline built into the language? If so, then it's not a prerequisite for popularity.
I dare say his essays are about starting up in general, and there is no secret plan to convert users into using Lisp. Every week on YCombinator Startup News, somebody asks about learning Lisp to do a web app, when there are way more other things to worry about than the language. Use something you want to use every day, not just in theory because someone else did. I'd learn an unknown language if I was at a startup that used it every day, but otherwise, I would default to using a web framework that I have already used in the past.
My conclusion is that if you're creating a web application using a web framework in any language you find interesting at the moment, go for it. Because that's exactly what Paul did. It just happened to be Lisp that he was an expert at, but it could have been any other exciting technology.
Whether Paul would use Lisp today or not on a new project is irrelevant as far as your plate is concerned. He could use Lisp, but it wouldn't mean that there aren't other good alternatives. And if he did use a language other than Lisp, maybe it would be so because he would want to learn web framework since he wouldn't be writing Lisp books any more, but Lisp may still be something he would normally use more often.
I think this is a great question.