This is a few months later, but... first, to answer the question: I don't know of a way to force the ports on the Extreme into stealth mode.
Next: Airport Extreme has not stealthed ports in a very long time. A search can find articles as far back as 2007 discussing this situation (and I have had AXs since that long ago, and can confirm this to be the case).
However... stealthing ports is like hiding one's wi-fi SSID: it IS another possible security action, but it IS NOT an absolute need. Some of the articles you might find in your search above state that disclosing ports is a standard in Internet specs. Either way, making sure that the ports are protected is most important, just as using the best (WPA-2) security with a strong password is most important for protecting your wi-fi network.
As long as your ShieldsUp! report shows no open ports, you should be fine. Just make sure to check from time to time - especially if you reconfigure the router, apply a firmware update, or make other changes to your network - to ensure that your ports remain closed to any connections you haven't specifically opened (such as port 80 for a publicly-available web site).
