While I agree with your general point, “are absolutely and clearly social conventions and have no reality outside of social convention” is overstating things. Differences in policies over time results in changes to the regions that would be clearly visible to aliens. (Guess where the border is: https://i.imgur.com/IW1JBAP.png)

No credit for saying “all signs of humans beings vanish”, the same logic applied to biology would suggest that life was a social construct, as if all signs of life were removed, there'd be no evidence of life.