Even before Mists of Pandaria was released and when people first learned that Garrosh would be the end boss, Metzen said in an interview that Mists of Pandaria wouldn't necessarily have to be the end of Garrosh' story. They did deliver on that.
I think the confusion simply came from one Blizzard employee saying Garrosh dies, when in fact he probably meant when Garrosh is taken down. You know, as in taking down a boss. His choice of words was just confusing.