Looks like I wasn’t the only one who took a while to figure it out:
[Y]ou can’t use the … AppEngineLauncher to use backends. So if you want to use backends … you will need to use the command-line version of the tools. Obviously this is a minor thing, but it took me quite a while to realize that was why backends were not working for me.
(and this guy used to work for Google … on the App Engine team!) Anyways, now I’m straightened out: