I also don't understand the managed version of Terraform. What capabilities will it be able to give you?
In what I do see value is managed Vault and Nomad. What do you think?
regarding managed Terraform: I don't have that much experience with Terraform, but you want GitOps integration and a state that has no race condition problems. As far as I know it's not too hard to build Terraform in your GitOps pipeline, but there's probably value when not every company need to do this manually for all their projects.