One thing that could give the benefit of the doubt to Comey: I'm not sure, but maybe someone else knows - the FBI can technically not "bring charges". They can only investigate, and DOJ prosecuters are the ones who tell the FBI that they have the evidence to prosecute or that they don't. Given that is accurate, Comey would have been told by DOJ, "You didn't bring us enough evidence to prosecute" (whether true or not).
He of course could have directly stated that he was told that, instead of the more ambiguous, "No reasonable prosecutor". Then again, maybe he wants to live past Thursday.
I have no idea, but in some ways watching Comey, it was almost like watching a POW give a forced statement while trying to send a coded message.