Their numbers look sound to me. Did you read the article?

>The ten most populous Bay Area cities listed above have a combined point-in-time homeless total of 63,527, and, margins of error notwithstanding, a census-estimated 92,800-plus homes vacant, a ratio of about three units for every two persons.

>The census’ annual American Communities Survey defines a home as vacant if there is either no occupant or a temporary occupant—temporary meaning “people who will be there for two months or less.”

>Vacancy rates are estimated by in-person visits to properties or by interviews via mail, email, or phone. The upcoming 2020 census will employ more exact methods and should result in housing and vacancy estimates with less room for error.