(Regarding the Headline) Surely you mean 35% of homeless who were tested. We can't say for certain that the sample is an exact representation of the general population of the homeless in Boston.
"Upon observing a cluster of COVID-19 cases from a single large homeless shelter in Boston, Boston Health Care for the Homeless Program conducted symptom assessments and polymerase chain reaction (PCR) testing for SARS-CoV-2 among all guests residing at the shelter over a 2-day period. Of 408 participants, 147 (36.0%) were PCR-positive for SARS-CoV-2"
The key thing here is they tested people from one single homeless shelter. Is this one homeless shelter in Boston representative of all homeless shelters in Boston? There is not enough info in this article to make that assumption. Nor is it enough information to make generalizations about the populations of the homeless who do not live in shelters.