But it is idempotent in this case. The server keeps track of your votes, so regardless of the number of times you do the same GET, the result is the same.
Hopefully the HN voting links never contain relation types which would imply prefetching! I recall that Firefox did some aggressive prefetching in the past, and some websites (maybe Wolfram?) temporarily blocked IPs which did the prefetching due to the extra load they were putting on the servers. It's very dicey, requesting URLs for a user which would have a low probability of being followed. Certainly, requesting tens or hundreds of URLs from a single HN page would be a reportable bug for either the browser (for wasting resources) or the site (for misrepresenting hyperlink attributes).
Regardless of all of this, if the HN server filters out duplicate votes, then the idempotence (is that a word?) of the request is satisfied, and it is OK to be a GET. [EDIT: after reading https://news.ycombinator.com/item?id=11308231 my point here is probably invalid, because voting is not a "safe" method. Voting alters the order of posts that future readers see, which is a material change for a discussion site. In contrast, something like a YT view-count tally doesn't effect a material change in the core material.]
Regardless of that, IMHO a GET request that changes the "fundamental" state of the world (not just UI state e.g. pagination/settings) is somewhat "rude" to begin with, because of the common expectation that GET requests are "harmless" and POST requests can change things. But like, that's just my opinion, man.
