I handled that flag and I did not think that comment an personal attack.
And seeing the 47 upvotes on the comment, I would be doing a bad service to remove it on a flag which is not clear.
To tell you truth, if I had seen the question when it was newly posted, I might have posted a comment like the one we are talking about, warning the OP that there are real dangers they might not have considered.
So even without all upvotes I might have left the comment.
If your question is why was there no reason given for denying the flag? We as mods can give a reason but most of the time I do only use that option if a person repeatedly flags for the same kind of problem which I do not see as a good reason to flag.
More as a 'please stop flagging for this reason' rather than explaining why I did not delete that particular comment.
PS, Adding a reason for handling flags on comments is not possible, only for flags on questions or answers.