They are saying that if those people keep it up, they can't have a Mod Team. It is an expression of an incompatibility between expansive power residing in the Core Team and the existence of a Mod Team. You don't really need a specific situation to see how that could obviously happen (classic power struggle), or to believe it has recently happened. Story as old as time.
At its height, this resignation constitutes (a) a plea for people in power to exercise it better, and (b) for those people to voluntarily become more accountable for some pattern of behaviour. (A) might be effective, if only because Rust governance so far prides itself on all the things that come with having a Mod Team. Not having one is embarrassing.
But point (b), which is indeed the focus of the resignation message, is plain magical thinking to my eye. Accountability I understand it is the acceptance of responsibility, by someone, for some thing. Someone else fundamentally needs to know what that something is for that to happen. It cannot happen if the thing is kept completely under wraps, but it can be approximated with limited but trustworthy disclosure, and this is the basis for the levels upon levels of that in e.g. national security regulation. That is a very difficult problem in its specifics, but the theory is simple: inform someone trustworthy and neutral, and then tell everyone else that you informed them.
What this message lacks is any indication of which people do know what the specific acts by Core Team members were and who did them, and what position those people are in to verify if anything is done about it. If you are unwilling to muck-rake in public, you need to give everyone else a proxy by which to gauge your generic claims. In normal governments this takes many many forms, including ministers, Inspectors-General, privileged parliamentary committees, etc. But you do not need a formal role, you simply have to nominate someone outside your group and your opposition (ie appears neutral on the face of it) that is aware of the facts. Without that, everybody who sees this will have to gauge your claims on the extremely minimal information provided plus your own reputation, but with no credible claim to neutrality on the issue. Even one such person would be better than all of the co-signatures on that letter combined.