I couldn't find the actual study, but I did find this quote about it:

In May, New York Governor Andrew Cuomo shared that in a survey of 100 New York hospitals and 1,000 hospitalized C19 patients, 66% of new admissions had been sheltering at home. “This is shocking to us,” Cuomo said, “this is a surprise. Overwhelmingly the people were at home. We thought maybe they were taking public transportation, and we’ve taken special precautions on public transportation, but no, because these people were literally at home.”

It might have just been a more informal survey like interviewing people during admission. In any event, the point is they assumed that people would be protected in their homes and didn't consider that one infected person forced to shelter in place quickly turns into multiple infected persons if they live with others.