HCL has a JSON representation [1], internally, objects behave that way. so it should be possible to write a Jsonnet wrapper around it. Terraform can currently parse json pipelines too.
No, it's not the same - you need a preprocessor. I do use JSON with Terraform via CDKTF although git diff is way more unreadable than with HCL. So, JSON is definitely a bad idea, but HCL v2 is not as good as the alternatives.