The MSM idiots may be carelessly expressing what seems to be the fact that the bomb went off prematurely.
It does not mean the bomber was not on his way to set off a bomb for terrorist purposes, but he may not have meant to set it off at that particular moment.
There is CCTV footage (I will try to find it again) which showed it going off as the guy was walking through a subway hallway, but it did not seem to be where he would try to plant it for maximum effect. There were people around but it “looked” like an accidental moment. Of course, that does not mean that the overall attack mode is any kind of “accident”....
Fortunately, the explosion was pretty weak as these things go. The entire hallway could have been consumed in explosion, with many people killed and injured, but it looked like the perp was the main victim, and even he was not blown all apart as often happens in the Middle Eastern suicide bombings. We may be very lucky that he was an amateur and not very good bomb maker, it appears.
Yes, I saw the footage you mention and agree with your well-written analysis.