I don't know why he would issue a statement that is a direct retraction of everything else that he has said.

I can imagine reasons - even assuming it's Bin Laden in the first place actually making the statement. It's not likely to affect how he seen by America and allies, but it might be a useful quote for waverers.

Also it lays down a basis for a defence, even if it turns out that the people who did it can be linked with his organisation - it would be that on their own initiative they went beyond what they had been commissioned to do.

As for anything Bin Laden had said which appeared to approve of killing innocents, the position would be that this clearly had to be understood in the context of Islamic doctrine about what is permissible in war. If he said anything that seemed to go beyond that, clearly such an interpretation would have had to have been a misunderstanding of rhetoric.

Please note, this is not me arguing that case, it is me imagining what the basis for a defence might be.