Keep in mind that speed of sound is not a constant in air, it varies by temperature and pressure (it's actually slower at altitude). Also, the initial fireball (of perhaps a few thousand feet in radius) would have expanded supersonically. that makes the calculation a bit more complicated.

Edit: Also, I watched the relevant portion of the video a couple more times, it seems like there's a possibility the audio is edited. The video cuts back and forth so it's clearly not a continuous shot, we're just trusting that they kept the timing the same and the audio the same, which seems like a bit much to hope for.