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very different story at StatNews source.
seems almost everyone tested was Latinx. the test with no positives was another test done earlier in a different area. you have to read it carefully:

28 May: Stat News: Testing blitz in San Francisco shows Covid-19 struck mostly low-wage workers
By Usha Lee McFarling
“When we proposed testing everyone in a census district, it was met with enormous obstacles. Where? Everything’s shut. Who? There’s no volunteers. There’s no protocols, there’s no supplies, there’s no labs. There’s no systems to get data back to people,” (Diane Havlir, who runs the HIV/AIDS division at the University of California, San Francisco) recalled...

She persisted. And in a matter of weeks, a hastily created coalition of hundreds of university, hospital, and community volunteers called Unidos en Salud pulled together one of the largest coronavirus testing studies in the nation. In a four-day blitz at the end of April, they swabbed and drew blood from 4,160 adults and children, including more than half of the residents in the 16 square blocks that make up San Francisco Census Tract 229.01. In the heart of the Mission District, it is one of the city’s most densely populated and heavily Latinx neighborhoods.
“What really comes out of these data is that low-wage essential workers are victims of this disease,” Havlir said. Many of those infected were working in food service, making deliveries, or cleaning offices despite shutdown orders. “These people were out working the entire time,” she said...
The first challenge for project volunteers was figuring out how to encourage a marginalized and minority population to agree to be tested...

The team brainstormed on how to reach people who had no internet access. They provided all information in Spanish. They leafletted. They went to bodegas and parks. They brought food, along with flyers about testing, to day laborers. They gave “golden tickets” to people who sheltered in the area but had no physical address so they wouldn’t be turned away from test sites. They rode around in cars with megaphones and P.A. systems. “We went old school,” Rojas said...
Jacobo, an affordable housing advocate who lives in the census tract and served as the Latino press secretary for Bernie Sanders’ presidential run, likened the effort to political work...
“Diane said this would require processing 4,000 samples in a very short period of time,” said DeRisi, a professor of biochemistry and biophysics at UCSF and one of the leaders of the CZ Biohub, a research center funded by Facebook CEO Mark Zuckerberg and his wife, physician Priscilla Chan. “I said, ‘Bring it on.’”...

In the days before the Mission project, the lab warmed up by processing in four days nearly 2,000 samples from another UCSF testing project in Bolinas, a semi-rural coastal community in Marin County, 13 miles northwest of San Francisco. The contrast between the studies ended up proving informative. In the largely white community, teams in tents tested 1,847 people who walked up or drove through in cars, bicycles, and “a really sweet Vespa from the 1960s,” said the project’s leader, Bryan Greenhouse, a physician and malaria researcher at UCSF.
Not a single person in Bolinas tested positive for coronavirus. ”It’s not too far away, but a world of difference,” Greenhouse said...
https://www.statnews.com/2020/05/28/sobering-finding-covid19-struck-mostly-low-wage-essential-workers-san-francisco/