Reddit has (and always had) this and many more. It's all part of their extensive anti-abuse system. For example if you go to someone's profile and downvote all their comments, that probably won't work. If you link your post on chat and tell them to go upvote you, won't work either. There's many more of these, and most of them look like they work but behind the scene reddit takes it all into consideration and knows people trying to game the system.

It sounds like he's saying that depending on the Referer header for a loaded comments section, HN may or may not generate upvote buttons that are duds.

Looks like there's an auth parameter in the upvote GET request, so sounds entirely plausible.

Upvotes from direct links don't count. It's an anti-spam technique.