However, logout can be extremely slow, and it seems to get slower if you bang on login/logout a lot; HN might have coded in some delays to keep users from slowing the server down.
My guess is, this user just didn't wait long enough for logout to work.
I "logged out" between viewing this comment and replying to it.
It's also broken for me on OSX Safari. I don't understand how I can be the only one with this problem on multiple browsers.
Does the problem happen for you at another location? Any new local routers/switches/access-points in the local mix? Does another OS on the same network (perhaps even via a VM on OSX) show the same issue?
Notably, when testing this, I discovered that logging out of HN from anywhere logs you out from everywhere. So even using a smartphone to login/logout might be a workaround for getting your OSX login session invalidated. (Of course discarding the browser cookies should work as well for the local browser.)