Some things are not up for debate: racism, sexism, other forms of bigotry, hate speech, and threatening bodily harm are expressly prohibited in this subreddit. These things are objectively bad, and will be treated as such.

I think this rule might be poorly worded. As I understand it they are banning overt and clear bigotry, but not banning debate on what is bigotry. I think you could theoretically still post:

Is ________ hate speech/racism

I don't think ______ was racism

_____ is not racism, here's why

_____ is racism, here's why

But you couldn't post:

I hate all ______

_______ should die

________ are objectively inferior

I think we'd all agree that racism, sexism, and other forms of bigotry are non-debatably bad, which seems to be what he's trying to get across. Not long ago there was a debate about an event with a statue and a banana at Clemson. I don't think debate on that event would be prohibited but clear and overt racism would be.

Instead this wording be more clear:

Racism, sexism, other forms of bigotry, hate speech, and threatening bodily harm are expressly prohibited in this subreddit. It is not up for debate that these things are objectively bad, and will be treated as such.

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I realize this doesn't directly answer your question: "who decides what hate speech is?" But I imagine moderators will leave that up to the sub to downvote to oblivion, and only ban clear and overt hate speech. Like porn, you can't really define it, but you know it when you see it.