Sorry, I think my post was confusing actually now that I read it again.
The text index and the $text operator are ONE thing. Atlas Search is ANOTHER new service available only in Atlas that offer way more flexibility and options.
The first can be used for simple text search and only supports a few languages. If you want more options, languages, text pre-processing (analyzers, …), etc, have a look to Atlas Search. It’s only available in Atlas because it requires more hardware and software contrary to the first option.