Your analogy would hold only if people with mental disabilities started trying to reclaim the discriminatory connotations of the word by using it to describe themselves.
And context here is people picked the terms for 4chan-humour value, and it's quite hard to argue non-autists lolling about how an instance of their behaviour is a bit like an autistic stereotype, or hehe trader is an anagram of how uninformed we think these trades are is an obvious candidate for the meaning being completely unrelated to the use of autist and retard as terms of abuse. Same as if I decide to categorise my friends using derivations of a four letter word: I'm doing so because it's a four letter word, and I don't get to play the 'but I meant it affectionately, specifically and non-literally' card if someone asks me not to swear in this establishment
I certainly don't think it's necessary for Discord to ban them, but if your jargon is chosen for edginess, you don't get to act surprised if someone decides it's too edgy.
This should be true, but recent events may prove it false.
A terrorist speaking in an encoded message to cause wilful harm is still liable for it, even if the words don't exactly say that.
Language is all about context.
If we were so strict about language, most Americans wouldn't qualify to graduate 3rd grade for their botched up spelling of British English.