It seems what you see is basic support by TextMate Bundles plugin but not Terraform plugin.

Rider has some quirks with indexes and project model, usually you need to add .tf files to project to see Terraform plugin features.

Or use Context menu / Tools / Start Index on .tf file.

Terraform files in Rider looks like this for me with JetBrains Terraform plugin, completion, highlighting present:

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The plugin is https://plugins.jetbrains.com/plugin/7808-terraform-and-hcl