I don't understand - essential workers still going to work also have to eat, and so go to grocery shops like other people.
IE: if you measured at workplaces that would capture only essential workers, if you measure at people's homes during work hours, you get people who mostly are quarantining, if you measure at a grocery, you might get a mix of both, etc.
But is specifically says
> and therefore not essential workers
It doesn’t say ‘not only’ - it says ‘not’.
"presumably" is the key word. They aren't saying "definitely".