Well, there are other statements that are allegded to have been made by him, where he says that he's innocent. And, besides, before 9/11, Osama was a insignificant player, he might've claimed responsiblity for the fame and following he'd achieve among the extreme of Islamics.
And, no, this is actually a very important issue -- because it invalidates all the reasons for the US invasion of Afghanistan. The Taliban were asking for evidence, which US refused to provide.