Thats been my theory all along - they probably have the rough master footage of the entire event, rather than the edits that appear in that youtube video. And maybe more from the Harvard days.
But -the point is, this probably isnt going to be some secret video - obviously who ever made the YouTube video was able to access it even before the 2008 election.
Im sure Harvard has some sort of media archive - so perhaps all it took was somebody actually taking the time/figuring out how to access it.