It seems as though he was not thinking rationally. He used both Tor and Guerrilla Mail, but did not consider sending the messages from a public WiFi hotspot? Seems like a really rookie mistake to make if you are going to commit a federal offense.
And, for all we know, if he simply didn't say anything, he'd have been OK, too. The only evidence in the affidavit besides his confession is "he used Tor".