But if you GET /posts/123 and then do it again, and in between, the author updated the post, you’d expect to get the latest version of the post, no? That doesn’t make it non-idempotent, because your GET requests did not change the state at all.
Not necessarily - plenty of systems offer no such guarantees, and the Web is by design eventually consistent [0]. This is what content expiration and various other cache control mechanisms are for - it's not always so important to get the latest version of a document. For example, the HN logo or index.html can probably be safely cached for days, since they are very unlikely to change, and even if they do, it's unlikely to have a major problem if someone only sees the new version after a few days.
[0] Note that, at the extreme, due to special relativity, there is no absolute notion of "latest version" on the scale of geographically distributed computers: it's physically impossible to say if a request made in China to a server in the USA happened before or after a change on the server, if they happened close enough together - order of tens of milliseconds, an eternity in compute time.
A command to add a user to the set of users that have upvoted a post would be idempotent. Because you can run it 20x and only the first call affects anything. A command to increase the upvote count for a comment by +1 would not be idempotent.