They don't just filter their e-mail platforms and consume hash lists from law enforcement agencies. On the contrary,
Microsoft funds and freely distributes the most common tool for checking photos (not file hash based, I think proprietary). That's likely the project the employees you mentioned were on, a job I wouldn't wish on anyone I know. I hope they were well supported both while working and afterward.
https://www.microsoft.com/en-us/photodna