I was too lazy to do the math myself, so I asked Copilot. Apparently it is plausible that Comey made a 9/11 reference. But wouldn't this be one weird ass way to reference that day? Who the hell randomly looks up the number of days since that tragedy?
If it ended up being some rounded number I'd understand. Like "hey, guess what, it's been over 9000 days since thousands of people died". That I'd understand. This... I'm more inclined to believe a conspiracy theory that he waited until 8647 days passed since a notable incident, so he could have an alibi.