It goes back to bbcode forums, in particular Something Awful, which is ~circa 1999 (1998?). Your post would have an "edited by [user] at [N time]" which only shows up if the edit happened after the next post, or Y time had elapsed. If you made an inflammatory statement, and got dragged for it, you could try and edit the original post, and it would show that you changed your position later, and people would drag you even harder. A lot of people will quote you if you make an inflammatory comment so that you can't change the past. Their quote won't have an edit tag and yours will, after their post, which further points to you backtracking on your original statement. HN doesn't really use quotes to any great degree, neither does reddit.

Reddit came into being ~2008-9 so this practice of EDIT: blah goes back at least a decade before then.