The defining characteristic about Pulumi compared to other tools is that it's not a transpiler, in fact. It's a multi-language runtime written in Go that can host many language plugins (Node.js, Python, .NET, Go, etc), as well as many resource provider plugins (native ones, OpenAPI-based, Terraform-based). So although yes it can use Terraform providers -- great for coverage across many infrastructure providers as well as easy portability if you're coming from Terraform/HCL -- it's not correct to say that it's "just using Terraform" or is a "transpiler".

Pulumi is open source on GitHub if you want to check it out: https://github.com/pulumi/pulumi.