I was the moderator who handled your flag, and I took a shortcut here. Basically, I agreed with your assessment that the comment is unfriendly and unkind, but decided that, as it was a small part of many problems with that question, a single-comment-deletion is not needed. It was a judgement call, and of course it could have been handled differently, or can still be handled differently if the community wishes so.
The comment was part of a very long thread on a question which was also off topic. The discussion itself was quite heated, and several of the comments on it struck me as potentially unfriendly, not just this one.
It was clear to me that the question should be closed, since it was both off topic and already causing unnecessary drama without actually helping the OP, plus generating lots of unsolicited opinion which doesn't really address the question - a phenomenon which SE sites try to avoid.
After closing, I considered what should be done to the comments. Deleting only the one flagged comment seemed not very helpful, because with the other not-nice comments, it would not have solved the problem as a whole. Besides, it contained not only the unfriendly expression, but also evidence that there are more people recognizing that the question is off topic. Going through each comment and deleting multiple of them would have taken much time and made the conversation unreadable. Deleting the whole conversation or moving it to chat would have been possible, but seemed pretty radical.
In the end, I reasoned that, with the question closed anyway (and presumably heading for the roomba script), there is no need to invest the work into pruning the comment conversation into something nice and pleasant. I don't expect many people to go into a closed question and read through a very long comment conversation. So I left it as it is.
