Isn't that kind of obvious to everyone at this point? It's just not out in the explicit open with the right kind of language. By giving such meager evidence of a "certainty", they're essentially giving away that they determined this via other means.

So, right now, let's assume it was a Russian spy agency that hacked the DNC. They're sitting there, thinking, "wow, these FBI folks somehow figured out it was us.". Then after reading this paper, "hmm, that's not a whole lot of evidence, they must have some proof that they're hiding. They must have a mole in our office. Find him, now."