Modnote: that 3 year old AMA has been posted and deleted four times now, please don't post it. Unelaborated links are not suitable comments here, nor is that at all considered a source by our standards. If you'd like to analyse the AMA as part of a larger, more substantial comment with more valuable sources you may. We're talking multi-paragraph comments.

No, it's never been cracked, and to my knowledge, no one has ever credibly claimed responsibility (I never saw the AMA that /u/caffarelli mentioned, so I can't analyze it). The actual content of the video was pretty much just unconnected nonsense, some of which can't even be reliably deciphered because of the poor sound quality of the feed. He did make what may have been an oblique reference to the WGN network (the bit about "I made a giant masterpiece for all the greatest world newspaper nerds" — WGN stands for "World's Greatest Newspaper"), but it'd be a major stretch to call that a socio-political message of any kind. My own theory has always been that it was just meaningless trolling/culture jamming, that they did it just to see if they could.

I'm not the right person to speak to the technical details of broadcast signal intrusion or pirate broadcasting, but it's possible to build a transmitter that simply overpowers the "official" over-air radio signal or even to splice directly into a physical carrier line. It seems that the perpetrators of the Max Headroom incident used the first method, since the first WGN attack was foiled when the station manager changed the broadcast frequency.

As with a lot of this "modern mysteries" type stuff, there are occasionally hints and innuendos by people who claim to have been involved or to know someone who was, but without meaningful corroborating evidence those can't be considered a reliable source.

The first intrusion was on HBO during a movie - "Falcon and the Snowman", and the Max Headroom guy complained about over-the-air scrambling of feeds (which was pretty new at the time). The one you are referencing was a follow-up or a copycat of the original incident.

There are quite a few transmitters around powerful enough to overpower the "official" satellite uplink, so I don't think it's really a mystery as to "how" this was done. Any credible organization would fire any of the employees who pulled such a stunt though, so it was no doubt done covertly.

EDIT: The original incident was the "Captain Midnight" hack, and was unlikely to be the same individual. The Captain Midnight intruder was caught and convicted of a federal offense.