Although it dignifies your question, I'd say it's because it's the first breath of fresh air in client-side development people have had in a long time. Having a real choice in that space is a big deal.
Do you have specific criticisms of CoffeeScript? That would probably have made for a more productive comment.
The answers which the question received, in addition to an insult, were that CoffeeScript removes bugs and mis-features, "is a breath of fresh air", and delights programmers by letting "the underlying awesomeness of Javascript shine through." Hardly real answers.
It is perhaps to-the-point, but I don't think it is aggressive to point out that CoffeeScript frequently shows up on the front page of HN. I am guessing that the comments about fanboyism and unfair down votes came as a response to a down vote, so the initial down vote couldn't be justified on those grounds.
As to the original question of the why CoffeeScript is so popular on HN, I too would like to know.
You are begging the question. You can't just say that CoffeeScript provides a real choice without explaining what differentiates CoffeeScript from Javascript and makes the choice viable.
I never got that behavior, there is nothing in the guidelines that said "Don't be aggressive and don't call people fanboys even if you think so and have something to show supporting that"
The closest that I can see in approach to comments is that you have to be civil and not say things in you wouldn't say in real life. The original post was not guilty on any of those counts. By no means was it an rude or uncivil comment, I see no name calling and he did support his statement in subsequent comments. If you are going to be a baby about it and can't take being called a fanboy then please accept that being called fanboy is uncivil and push to include that into the approach to comments.
Do you have specific criticisms of CoffeeScript? That would probably have made for a more productive comment.