Is there any possibility they are under formal legal contract ie. NDA? Don't know how formal the Rust organization is setup/whether that would be a part in the process of joining the moderation team.
Is HR on the employees side? No. Same thing here, the moderation team didn't realize that their job was to protect the core team from the rest, holding the core team accountable wasn't a part of their job even if it was warranted.
An NDA for an open-source project? I sincerely hope no one tried that, but if they did, that's a radical idea and the effects must be studied (never let someone doing something weird go to waste, science can learn from it!)