Look at the timeline:
- Sessions didn't recuse himself until the beginning of March (Mar 2nd)
- Sessions didn't officially announce that this recusal
extended to the Flynn investigation until late April.
- Comey didn't know he was going to be fired (May 9th) until minutes before it was officially announced.
- Mueller wasn't appointed until May 17th, a week afterwards.
If he knew something, who exactly was he supposed to tell in the DOJ? Or in Congress? There are supposed to be "checks and balances", but those are just words.