Harvey's presence on the show is sure to hold a ton of promise for Season 4, especially because it means that Claire officially has someone in her corner to help her get a leg up on her husband (which, spoiler, is going to be very important to Claire's #goals this season!).
Independent clauses can stand alone as sentences. To be independent, a clause must contain a verb and asubject and not begin with a subordinating word or phrase.
The example is right, it's the statement that A dependent clause (or subordinate clause) cannot stand alone as a sentence. In itself, a dependent clause does not express a complete thought that is wrong.