Dodge?
How does that statement imply that pain is the only acceptable case? There are many other horrible, distressing and unpleasant things in life and not all of them involve pain. Pain is just a simple case.
It's a dodge because it tries to use a loophole in the rules about killing. You can't intentionally kill, but the people who you want to intentionally kill suffer pain, and you're allowed to incidentally kill in the course of relieving pain. So you just characterize the killing as incidental rather than intentional and presto, you can now morally kill them.
The loophole doesn't work for Alzheimers because it doesn't cause pain like that.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.