People seem to be talking about two different things interchangeably:
1. The amount of damage of the explosion at various altitudes
and
2. The trigger mechanism (human / timer)
Just because the explosion may not cause more damage at a higher altitude does not mean that if it was human triggered the person who did it knew or thought that. All other things being equal, I would expect a suicide bomber to wait until a higher altitude and better location on the assumption more damage would be caused. Flight being late and low altitude to me suggests a timer.
On the other hand, if human triggered, maybe the bomber thought good enough, maybe so nervous thought it was at cruise altitude, or maybe just made a mistake. I suspect they will figure it out at the end though.
TME