I might have this wrong, but my understanding is that his plea deal only covers state prosecution, but not federal. It is possible the feds could prosecute him, but I haven’t seen nor heard if there is a side-deal to keep that from happening. Surely his lawyers would have tried to secure that, but did they succeed?
Has he been charged at federal level yet? Would imagine that federal prosecutors prefer to negotiate deals separately, but who knows what strategic arrangements go on behind the scenes.
Even if this won’t touch the federal case, it would be worth his while, purely because state prisons are far gnarlier than federal. He’s still likely to get some credit for having eventually cooperated at state level.