Because he then proceeded to log onto 4chan and tell a story with racist slurs. I don't care that he wished the black death on their mothers, good for him, I care that he's racist while writing this post.
I had to go look and I'm still not sure I understand, but it seems like it's derogatory because whoever wrote the greentext is basically calling Indians "street shitters".
Yeah, but isn't the black death thing infinitely worse? You just gonna sit there and decide which bad things are bad enough to matter?
Here's the thing. This dude is butting heads with those dudes and they're gonna insult each other with whatever makes the other most mad. Here, that's slurs and black deaths.
I can agree it's a little bit racist and wrong to tell the story using that sort of slur, and honestly I kind of think likely the entire story is very racist due to being fake and made up to get a laugh at the expense of Indian people, but if it's real it seems worse to be knowingly bringing harassment down on your coworkers by intentionally antagonizing people for a laugh when you're supposed to be doing your job, I think that probably does more actual harm.