The virus affects young and healthy folks without co-morbidities dramatically less than it does old folks at a nursing home. Based on New York City data, people without co-morbidities account for something like 6% of hospitalizations [1]. Old folks on a cruise ship full of old people, about 20% showed no symptoms. Is it a big stretch to think that scales to prisoners as shown? They're pretty young (only 2.8% of prisoners are over 65 [2]), and therefore pretty unlikely to show symptoms let alone require medical care. Age is unquestionably the biggest factor in outcomes for COVID [3].
"Asymptomatic" is kind of a sliding scale. Are you sick? No. Do you have a stuffy nose? Kinda.
[1] https://www.the-scientist.com/news-opinion/nearly-all-nyc-ar...
[2] https://www.bop.gov/about/statistics/statistics_inmate_age.j...