Perhaps they were, but that has nothing to do with the FBI investigating WHO attacked the DNC server and more importantly WHY.
When the DNC emails that were hacked by Russia started leaking out the inference was pretty fucking obvious.
So I would imagine that that's when the logical questions started being asked like: Did the Trump campaign know about it, and did they offer something in return for Russia doing it?
Which essentially is what the investigation is all about.
The dossier may or may not contain relevant facts, but I think it's quite critical for the American people to know that their President isn't beholden to a foreign power, don't you?