It's a win for those who wouldn't negotiate because Reddit will have to estimate fair market value if they want to hire someone.

Instead of talking the first low ball offer, they'll get what reddit thinks they are really worth. Which is still probably lower than what they could have negotiated.

I think in this case they mean new to the new company, not new in the sense of Jr employees. The best time to negotiate your salary is probably when you are first being hired.

Right, but that creates a compensation differential between people who are comfortable negotiating for more money and people who are not. The point of this no-negotiation policy is that it eliminates a bias toward rewarding negotiation-oriented employees more than non-negotiating hires, despite the fact that they are doing no extra work and creating no extra value for the company. The company has to present a competitive offer up front if they want to attract good staff.