Hi Renier,
This is correct. At the moment, Gophercloud (the Go OpenStack SDK that Terraform uses) only supports authentication through Keystone.
For Terraform to be able to use a Swift-only environment, first Gophercloud would need support for something like swauth. If there was another Go package that implements swauth, it might be possible for the two to coexist nicely in Terraform.
If this is something you'd like to investigate further, please open an Issue on the Terraform Github page. :)
Thanks,
Joe