I think we can assume the HN poster is well aware of where they first saw this submission link.

Even if they did (and I dont see any evidence either way), lobsters is an entirely different site (that also requires invites, excluding the majority of users from commenting).

Since the other person decided to not post it on HN I dont see the harm in someone else reposting it here. Likewise if someone discovers something neat via HN they might repost it on, say, reddit (or vice versa).