So, it's fast because it uses Pulumi... Does Pulumi no longer build on Terraform providers? (EDIT: looking, it seems like many Pulumi providers are still terraform wrappers...)
Either way, it feels like this is just a Pulumi feature.
Not just a feature of Pulumi, its just used as the IaC of choice for the out of the box deployment providers. This can be replaced with anything.
At its core Nitric is really a tool to collect infrastructure requirements from an application and those requirements are delivered to a server that is built using the Nitric deployment APIs.
The exception are native providers which use new stuff like AWS Cloud Control API.
You might want to look at the repo I linked, check out the source, or hear it directly from the folks at Pulumi if you're interested.
Nitric infers the necessary cloud runtime by building a spec based on the resources you've imported into your application code.
At deployment time Pulumi is used to provision the resources (because we think it is an awesome tool).
Note that we could have also opted for any other IaC like CDKTF, and that you could either modify our providers or implement your own providers using your IaC of choice.