I ran into the same issue; I believe this occurs mainly when running tofu validate on a project that was initialized with terraform init. However, IMHO, Terraform users should not be forced to use OpenTofu and vice versa; perhaps a better option would be to make both terraform-validate and terraform-format hooks support both Terraform and OpenTofu, or to add separate hooks for each.

Regarding the first option, I believe something similar to #419 should work within the scope of the terraform-validate hook. This should give users the flexibility to use Terraform or OpenTofu:

hooks = {
  terraform-validate = {
    enable = true;
    package = pkgs.opentofu # or pkgs.terraform
  };
}