(Since there's no other definition of historic that I know of (500M years seems as arbitrary as any other number), I think the person was referring to the liter

(Since there's no other definition of historic that I know of (500M years seems as arbitrary as any other number), I think the person was referring to the literal definition of historic, namely, written records. Not that I think this is a very fruitful argument to make, or whether weather recorded in shells could be considered written is perhaps also debatable)