It's not that misleading even if the distribution is not perfect. It would be extremely unlikely that not a single person is innocent.
Given that this is a jail and not a prison, these people are mostly awaiting trial. From a certain point of view (innocent until proven guilty), most of these people are innocent and they can't even get a letter.
True, but I think that misses the point. I think the comment was an attempt at facilitating empathy for the incarcerated who are in this situation since for most people they don't see themselves as criminals (even those who are in fact criminals) and it is harder to empathize with those who you see as being part of a different tribe/group/class.
The fact is that these are fellow humans who are suffering, not because we decided as a society that this was what justice means, nor because this was the result of some study that figured out this would decrease recidivism, but just because some for profit institution decided they wanted to do something that would directly result in them suffering. That is enough for me to support this lawsuit.
Saying "at least 4" is pretty misleading here, since you're assuming the 0.5% is perfectly distributed. That estimate could be accurate without a single innocent in these specific 850.