A weird theory that has been on my mind for a while now, well I know it probably sounds ridiculous/not feasible, but might as well post it while it's still fresh.
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Could it be possible that some people survived the initial petrification wave by living in bunkers? Like, we already saw Byakuya and the other astronauts survived by being in space at the time, which is realistic. But would the reverse also work in this case? From Senku and Xeno's discussion(s) in chapter 171, they said that whatever the petrification ray was, it was a form of matter that was being pulled by the earth's gravity. But despite this, I think there would still be a limit on how much it would penetrate the ground. Especially considering bunkers are supposed to offer protection against world ending/catacylsmic activities.
With Byakuya and the other astronauts, I think sensei is trying to make their survival somewhat poetic/romantic (Like, they were the last of their kind). But I do think bunkers can work in such a setting imo.
TL : DR - People who live in bunkers could have survived the initial petrification wave. Though where those bunkers would be located is another issue altogether