I'm not quite up to speed on the current operation but on and off over the last year it (like many other US services) have been operated as a freight only service.
Also, given all the current travel restrictions, who are all the passengers travelling on this flight,
Don't know abut the specifics of the SFO flight but many countries still allow entry by air to certain categories of passengers.....for example diplomats, some medical professionals, sometimes own nationals repatriating, forces personnel, essential workers in certain occupations etc etc etc.....a careful the US state department/UK gov websites should help you track down the details.