[lamontcg]

The genetic clock was estimated from bat coronavirus mutation rates in the wild, not from SARS-CoV-2 mutation in humans.

And virulence != transmissible. They got sick by inhaling everything that was floating around in that mine. To get more people sick you'd need to take them all to that mine.

Recombination to produce a viable and more successful virus is also very rare, that is unlikely to have happened to any of those miners, much less several of them (over populations it becomes likely but there we're talking many millions of infections as well).