Not quite - if there was no patents file, the BSD license would have a implied grant associated with it which would not be revoked by the lawsuit.
The "implied" license doesn't have a strong legal finding backing it up yet. Even so, most lawyers are comfortable with the idea that if a company tells other people to freely use their software, they can't then come back around and sue those same people.
I.e. the exact situation you would be in if you sued Facebook and there was no PATENTS file.