Terraform is not a language. HCL could be considered a language, but really requires the additional context of the Terraform resource model and the individual schema pertaining to the resources under management to decode. It populates an internal resource model, before changes are effected by the Terraform core engine.

One can configure the self same resource model using JSON, which can be trivially generated - as you correctly point out - using whatever language and paradigm you like.

However, since the language doing the generation does not influence the order of operations when applying the effects, the model is declarative. This is identical to the CDK configuring the CloudFormation engine via YAML. Indeed, there is a CDK for Terraform too...