Replying to myself, but it looks like this “story mode” will actually be just a scenario where you run and and delete the final boss, not the co-op full raid clear that I was hoping for. Unless I have misread this blue post.
So in essence it is a slightly more interactive “click here” button to see the ending cenimatic.
Shame on me for getting my hopes up. Should have known better.
Oh and also, it’s going to be released along side the final wing of LFR. So you know… weeks after its already been spoiler’d on YouTube.