Well, the location of that fireball sure seems to line-up with the location of that mystery bag.
Also, it looks like all the runners had just started squeezing to the other side of the street to make space for rescue efforts. They're lucky they did, because that second explosion's proximity would have taken a lot of them out. A few seconds earlier, and it could have been even worse.
Yeah, the first frames of the explosion in the video seem to centre on the kerb as well, but it doesn't necessarily mean that was the bag if they think they were black bags.
The table and chairs are relatively unscathed, maybe as they were surrounded by people. If the bomb was further back you might see more damage to the chairs and the black fence that surrounds the restaurant area? The Mailbox could have been left relatively undamaged due to the direction of the explosion.