I wrote a comment under an answer, claiming that the answer was wrong. The poster replied to me in another comment, showing that my argument was wrong. Acknowledging this, I deleted my comment and then flagged the poster's one as "No longer needed". To my surprise, the flagged comment disappeared immediately, and my flag got marked as "useful", seemingly without any moderator acting upon it (or the author of said comment). Why did this happen? (If it matters, the question, and therefore the answers, were all "community posts".)
Edit 01.01.2018: Following some suggestions in the comments, I asked the poster whether he deleted his comment himself right after I had flagged it. No, he didn't, so the oddity remains.