It's not true, and what you describe is correct. I can only assume that the author has used some provider that hasn't implemented this properly (helm, I believ

It's not true, and what you describe is correct. I can only assume that the author has used some provider that hasn't implemented this properly (helm, I believe, is one example), or that they've run into one of the cases where terraform treats configuration/attachments to a resource as a separate resource (e.g. IAM role vs attached/inline IAM policies).