I’m sure someone will pull the relevant tax laws (it’s 6am and I can’t be bothered searching for them), but the US has arrangements with many countries (my own included) which allow non-citizens to work in one country and pay taxes in their country of origin.
US exchange teachers working here - for instance - don’t pay Australian taxes. They pay their tax to the IRS and the IRS and the Australian tax office sort out what portion belongs to whom.
It’s highly possible that the person mentioned is working under just such an arrangement, and not doing anything illegal at all (I can’t imagine a university paying him cash, which is what he’d be looking for if he was trying to buck the system).