Concur: the writer wasn't very good. About clarifying anything!
Sound from an explosion or thunder is right at 5 secs per mile.
7 minutes? Maybe 7 seconds. Even two minutes is almost too far for anything less than a volcano-sized boom.
7 seconds would mean about 2.3 km away. The blast of the energy alluded to would have hurled him to the ground at that distance. Tunguska blast (about 16MT equivalent) knocked people off at what - 400km?
True, but something "comparable to Hiroshima" is larger than most volcano sized booms. I do believer you could hear a Hiroshima sized bomb 90 miles away, and it might sound like, or at least as loud, as a stick of dynamite a klick away. I would think it would be somewhat lower pitched and longer lasting though, just like far away thunder sounds different, as well as being softer, than nearby lightening. In this case, the major explosion may have occurred in the atmosphere, with only a small chunk hitting the mountain.