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Not understanding is fine: assuming that something one doesn't understand must be a slur is not fine.

He didn't assume but presume. And I think that's quite understandable for a non native speaker, given the connotation of the word 'dark'.

Actually in this case presuming is not understandable, there is no evidence or reasonable grounds to support his statement.

We are on the internet, and online dictionaries exist. There is no reason to presume or assume the meaning of a word when it can merely be looked up.