That doesn't matter from a staff point of view though. You have a queue to work through. You'll be putting in an 8 hour day dealing with the stuff the system doesn't catch. The automation just means they don't need as much staff.
You're still going to get false positives and false negatives that need human review, and at a scale of Facebook, that's a lot of humans.
Not dumb, but I do think it's not thought-through. You're proposing a simple solution to what is a hugely complex problem, and throwing ML at it just isn't going to work. To get "plenty of training data," a human still has to classify all of that, leading to the problem of viewing that much unpalatable content by a human. You also have to train your network, which requires humans to verify the accuracy of training, hence viewing the content again.
If it were as easy as text-based spam filtering, this wouldn't even be a discussion.
How does it differentiate between normal ranting and hate speech? How is hate speech classified in the US vs Saudi Arabia? How does it tell the difference between someone asking a child innocent questions and asking them sexually-related questions on camera? Does the algorithm get trained to flag videos about depression that might lead to suicide, or does it say they're supporting getting help FOR depression and leave the video?
What about subjects that aren't already in the corpus of "flag these naughty things"? You still have to get a human to look at those; most likely a data scientist who knows what the algorithm is doing and what needs to be done to correct the training. Machine learning as-is will not get us there, so in the meantime the only option is moderation by people described in the article. It can be outsourced elsewhere, but it's just shifting the responsibility to a different subset of people.
https://news.ycombinator.com/item?id=22487403
https://news.ycombinator.com/item?id=5068626