Well, considering that he wasn't discussing anything to do with the PC's on the network (not to mention that Norton Personal Firewall will not do anything to protect your Wireless network from being sniffed and potentially infiltrated), I don't see how this is relevant. I suppose with you software firewalls, you're relatively safe from having your individual PC's directly attacked (but the network traffic can still be sniffed), assuming that you don't have other glaring security holes open on your computer. NPF doesn't protect your computer from weak security, but it is a good complement to strong security.
The issue here is that he was able to get access to the router in question and could be using your wireless bandwidth to do anything he wants to do (including illegal activities that can be traced back to your ISP account). What if he decides to run his 20,000 song Kazaa collection from your router without your knowledge? When the lawsuit comes in from the RIAA, you're probably screwed (though, they have no evidence that your computer has ever had the files on it, you can not disprove that it is your internet connection being used. A reasonable deduction would be that you downloaded them and you'd have to have MAJOR proof that your network was hacked to convince anyone otherwise).
I think what this article proves, beyond a doubt, is that security can only keep honest or naive people out of your network, especially a wireless one. Nevertheless, the measures that he was able to circumvent in his article, provide a speedbump for the casual browser... if there's a wide open wireless network next door, they probably wouldn't bother with yours. Kind of like why burglars jiggle door locks to see if the door is already unlocked. Why bother breaking in when plenty of networks are wide open already?