Hacker News was launched 2007.

XMLHttpRequest has been around for about 7 years then and using it was called Ajax for about 2 years already.

It is strange. I wonder if it's an approach that lets duplicate vote attempts get automatically responded to by the webserver cache rather than passing through to be detected computationally at the application server level, which would also involve a DB call.

Conceivably this would make the architecture more robust against vote-spamming attacks, and the approach would also provide an enhanced version of this benefit if using an external caching reverse-proxy service like CloudFlare or similar (though I believe HN does not use CloudFlare).

This is complete supposition on my part.

There's also an XMLHttpRequest wrapper for when they care about the response.

This is used for write-only GET requests (with graceful fallback when JS is disabled since those are plain anchor elements).

Probably. I wonder though if they are trying not to use any libraries and Image was just easier than xhr.