Well, in office there was very strict no smart phone policy. Privacy laws were big issue when WFH started in march. This company also does phone support for other clients, and handles credit cards. There are huge fines. I am not sure how they made it compliant for FB. Propably sensitive stuff is still handled from office.
They certainly hide also your personal information like name, peofike picture, etc... So at the end its just moderating texts or pictures without an author? Maybe they also automatically replace names in text?
Child abuse pictures/videos can still be shared or sold online easily without PII. Other exceptions are celebrities or the difficulty in identifying PII in text.
Couldn't you just watermark all photos before showing them to workers? Probably wouldn't stand to sophisticated attacks but would guard against theft from "low-skilled" employees...
But if you have privacy expectation on fb...