Interesting, I wondered if the same can be done on hn. Likely easier since the IDs appear to be sequential. But one can simply edit their post to point to the URL too if they wanted, so it defeats the fun of the exercise.
Unfortunately, I wasn't allowed to keep the karma :(
That surprising to me. I think you should keep it as you did something novel. At the very least as a 'bug bounty'.
In a similar vein, Reddit's "self" posts were originally formed by users submitting posts linking to themselves, by guessing the URL for the next submitted post.