When I'm working around live wires I will frequently test the circuit, trip the breaker and then re-test the circuit just to be sure. And even after all that I still will occasionally brush wires to frame to make sure I haven't over looked something. I'll readily admit to a bit of irrationality where all that is concerned.
That said I can't imagine buttoning up all that Rube Goldberg contraption, transporting and then setting it to armed without a lot of trepidation that it would just go boom. Maybe the tilt mechanism got stuck in the contact position, maybe there was a short somewhere, maybe maybe maybe.
I'm really curious what his heart rate was the second he threw the switch. Did he have 100% confidence in the design or did he flinch.
He might have set up a series of latching 'fail-on' and 'fail-off' relays to ensure that the last switch could arm the bomb, but would not cause detonation. The article reports that the box made some noises; perhaps he set it up so that arming the bomb resulted in a certain noise that would confirm it would work.
He might also have performed continuity and voltage checks before screwing on the last booby-trapped panel. He certainly took some time in the hotel room to get it properly set up.