In your case you do not need inline deployment at all, but you need environment deployment. Please read the difference here. So you simple need to delete your existing inline deployment from that YAML and create new one at https://ci.appveyor.com/environments. Then you will be able to deploy artifacts from any successful build manually.
This button simple allows you to select environment to deploy artifacts from build you open. It can be also not latest build, but any other successful build you select from the builds history.
If you go from some specific environment, UI will allow you to choose project and them specific build from its history.
Ok thanks for being precise though still not clear about the thing that I was asking. You are also (I think) saying that the fact that the deploy button *is usable* is due to environments, not the appveyor.yml configuration.
Exactly, this button links specific build (it's artifacts) to environments (which are being configured in UI, not YAML). I think if you play with UI from both sides (this button and also "Deploy" menu for specific environment), you can get the feeling how builds and environments can be glued together.
Main idea behind environments is to decouple deployment from build so you can deploy the same bits (specific build artifacts) multiple time at any moment, without re-building.