Horowitz can only report the facts that are already public knowledge. The FISA warrant was based on the illegitimate political opposition research from the DNC and was not verified. Its hard to argue that such egregious fraud is just “sloppy” or a “bureaucratic screwup”.
Barr is under no obligation to agree with the conclusions of the IG especially since he can apply the findings of Durham that have a much wider scope than Horowitz has. If Barr says its a crime it doesn’t matter what Horowitz might think.
See #45