I also see those (mostly when using hnreplies.com, but always from the same posters.

I assume it's because they have written something they wish they hadn't, and while they are drafting a new comment they are replacing the original comment with that placeholder.

Just my assumption, no inside info.

(The earlier flagging was obnoxious, but your over-the-top accusatory style will certainly garner flags, as well)