The reason for the NDA is obvious: Facebook doesn't want you to know that there are people doing this job. Facebook doesn't want you to even realize that this job needs doing, they want you to think that the platform is just clean and nice without any of this icky stuff.

It's part of what makes Facebook enjoyable for end users, and that they've managed to do: keep the platform clean of most awful things, but it comes at a heavy cost. People don't notice this cost, and that's by design.