Apparently he later changed the last line from "This wasn’t meant to be a malicious hack, but rather a simple social experiment to see how much traction/investor interest I could get" to "LEGAL DISCLOSURE: I have neither attempted to read or plan to read any voicemails," in the version of the cache I have, which is a telling edit.
With lots of providers you only need the PIN if accessing voicemail from a number that isn't your own, eg "dialling in" to your voicemail from a payphone/friendsphone/etc (dial your own number, press *)... if it detects it's from your own number, it generally just lets you in without the PIN.