Yes, depending on non-renewable fuel, by definition, means that at some point, the fuel will run out. Ridley only points out that naysayers rely on a hidden but faulty premise: that the future will resemble the past. Yes, we will run out of fossil fuels if we keep using it, but whose to say that we will keep using them? ... these folks' error lies in assuming that future ways of production will resemble past ways, and time and time and time again, this assumption has proved erroneous! Ridley's point is that while we can NEVER say that the future WILL solve all pressing problems, so far we have. And we can assume we will in the future because our method of exchange has globalized the "collective brain," assuring that innovation will keep occurring and the best minds will all be working on the pressing problems of the day.But what if the best minds are convinced by this very argument? Then they will, rationally, cease to work on solutions to these types of problems. So Ridley's argument is only true to the extent that people don't believe in it.
This works on an interpersonal level too. If you truly convince yourself that a situation will end up fine, you have zero incentive to do work to alter it. Thus, in the case that you are not worried at all about a situation, the situation will likely not turn out fine. This has tons of implications... but for tonight I'll just say that it's a cool paradox.
* 95.9% of people agree that, instead of reading the book, you mindswell just rely on Kevin Currie's review to draw sweeping inferences.