I don't see why its so far fetched that a republican staffer who is anti SpaceX would leak a report (or contents of it) to an anti SpaceX journalist. Northrop is trying to save face so of course they are going to point the finger at SpaceX.
Also, Northrop didn't point any fingers at SpaceX. They can't even say anything about the mission as it's classified.
When you lose a billion dollars you are pointing the finger at someone else. This is true in general business and especially with government money.