I would assume many of these testimonies may be a bit misleading. Most missions flown by B-29's generally flew at a very high altitude (above 30,000 feet), making it difficult for the gunners to "strafe the population." Additionally, the firebombing missions which encompassed many of the flights over Japan, were known for their tremendous and uncomfortable heat, making it nearly impossible to fly at low altitudes to strafe civilians. The pilots also had difficulty flying at lower levels due to violent updrafts which occurred from the intensity of the fires below.

However, it may be possible that smaller American aircraft strafed these people. By the end of the war, aircraft carrier strike groups began to bomb Japan's harbor's and industry. Thus, they may remember being strafed by planes from aircraft carriers, not B-29's.

On another note, do you have access to these testimonies? Is there a link? What was the documentary name? Who made it? This question sounds interesting and I would like to continue to investigate and learn.

By August 1945, most B-29s no longer had their machine guns installed, so they could not have strafed anywhere even if they'd wanted to. Even if they still had their guns, it would have been beyond senseless for the Army to allow their $600,000 airplanes to come within range of small-arms fire.

However, both the Army (from bases on Iwo Jima and Okinawa) and the Navy (from the carriers of Task Force 38) launched tactical raids on Japanese cities through the end of the war (including after the Hiroshima and Nagasaki bombings). I think it's unlikely that either service launched raids targeted at Hiroshima or Nagasaki post-bombing, as those cities were on more or less of a "hands off" list since they were being reserved for the atomic bombs, but raids might have redirected there because of weather.

And it's entirely possible that survivors would have been transported to other cities after the bombings and then been strafed by American planes there (the key being that combat operations against Japan did not immediately cease on August 9th). There was (at least) one Japanese man who was in Hiroshima on August 6th, for example, who was then taken to Nagasaki where he was present for the bomb dropped there on August 9th.

I don't think in any scenario that American tactical pilots (or pilots of any other combatant nation, for that matter) would generally have intentionally strafed civilians.