Well, sitting in front of my computer, I actually heard the third pressure cooker bomb detonate off in the distance (about five miles away, during the battle of Watertown, in which the elder Tsarnaev became a speed bump for the younger's temporary escape). I had been paying attention since the report of Officer Collier's murder earlier in the evening. The chase was all but certain to be connected to the fleeing brothers.
But you are right. #6 is not a bomb unless loaded with explosives hooked up to a credible detonation system. The brothers apparently used a 2.4ghz model car remote rewired to energize a Christmas tree bulb carefully broken in such a way as to ensure the filament remained intact and inserted into the explosive material, which was extracted from fireworks legally purchased in nearby New Hampshire.
Note that the Tsarnaev boys didn't use a stinkin' clock, digital or otherwise. The used a well-engineered remote control, which was highly resistant to spurious RF. That's important if a terrorist wants to avoid work accidents in an RF-rich environment, such as downtown Boston.
That was clever of them. It still doesn’t cast suspicion on all pressure cookers, and for the purposes of the test, since we can’t see inside the pressure cooker, it’s just a pressure cooker, not a bomb.
The test should have included a U-Haul truck, a la McVeigh et al. It *can* contain a bomb, but is not a bomb.