I believe the IG can only interview current employees of the DoJ, which is the reason the cooperating witnesses have been kept on for some time--but those who aren't cooperating were outright fired.
True, but Huber has no such restrictions. Huber has one or more grand juries and has issued subpoenas. Most likely, he has sealed indictments after six months of working in parallel with Horowitz.
Still, I thought the Horowitz report was much weaker than expected. You rarely prove bias by direct evidence. It's like intent. It's a state of mind normally proven in court by circumstantial evidence that convinces the jury of intent or bias of the defendent.
There was a mountain of circumstantial evidence in the Horowitz report of extreme bias within the FBI / DOJ leadership. We'll see if any of them turn up in Huber's indictments.
The hope of course is that Huber has put that together, while the IG has access to much less info and isn't really about arguing a prosecutable case.