Config is currently only managed per-project, you can't use it to read another projects configuration at the moment.
The namespace is so that we can tell the difference between like "aws:region" and "my-project:region". "region" alone means "my-project:region".
If you want to read another stacks config the currently suggested way to do this is to make the other stack export the config parts you care about as stack outputs and then use a stack reference in the new program to read them.