For what it's worth, I have personally seen xbox live make this mistake before and provide a sneak peak of content yet to come because some data entry person hit the wrong ****on. The OP doesn't post, but has been around since 2005. I doubt a troll would gestate an account for 7 years just to troll. So I choose the believe what the OP saw and am looking forward to some of this DLC at some point in the future!
I heard awhile back from a eb games employee that Pearl Jam and Judas Priest were coming soon and one week later there was the Pearl Jam Songs so I think it's a safe bet to say Judas Priest is one of them. I heard Rob Halford jumped at the chance to be on Rocksmith.