There’s nothing in that video that confirms that he was Suspect #1. Also story has it that Suspect #2 backed up over his brother Suspect #1 while trying to escape.
Sounds to me (and I speculate) like there was a shoot-out between police and the suspects but the suspects were not in clear sight of the police (at least not the car)and this gave the younger brother time to back over his brother and leave the scene. I would assume that the police didn’t even see the car when it left and probably waited a while before approaching not knowing that one suspect was down and the other had left the scene.
There were lots of people taken aside and questioned that whole 24 hour period. Remember the guy on the ground face down? One could have assumed he was also one of the two bombers, but he was just a passer-by.
My guess is this naked man was suspected to be one of the suspects like many others were that confusing night.
It could be the guy whose car they hijacked. They let him go after about a half hour.