Sure, but Firesheep made it possible to exploit it without even "script-kiddie" level skills. No easier in principle, perhaps, but easier in practice.

The point is the type of security scenario being talked about here is wildly different than the one made simpler to exploit by Firesheep. This was not security through obscurity. If your servers are configured correctly, unguessible filenames are just fine from a security perspective, at least as secure as passwords.