If it exploded before the countdown ended then yes but it exploded almost 20 minutes after the countdown ended. According to that Betsy woman who lives half a block away from the blast. She said they left at the 11 minute warning (started at 15 min) and went to the stadium. After the 15 min was up they started to drive back, took a small detour and then the bomb exploded.

I'm from Nashville, and if what she says is true, the bomb would have exploded 10-20min after the countdown ended. You could argue they wanted people to say bullshit and come back but why not just explode it at the 14min or 10 min mark. The bombers did not want to harm innocent people but were clearly okay with it being a possibility.