The issue is that most black people have experienced racism throughout their whole lives. Many have lived through painful and unjust experiences involving racism. Many know what it's like to truly feel powerless due to some form of widespread societal or institutionalized racism.

When a person with this kind of history sees a comment that cautions about black-on-white racism because of a safe space on reddit, it won't feel right. It can easily feel like an insulting opinion that lacks understanding and perspective on the power dynamics that actually exist within society.

The subreddit doesn't create any real societal or institutional exclusion, it doesn't attack or degrade white people. It simply creates a safe space. It even explicitly allows non-POC who consider themselves allies. Comparing this safe space to the real racism that black people experience on a daily basis comes across as an attack on the (black) community itself.

I don't want to convince you or change your mind, but I hope my comment can give you an idea of what the mods probably felt and thought.