What's in a word?

Black Panther - the latest superhero movie from Marvel is being lauded for bringing "diversity to the big screen".  I haven't seen it and it may be a great film or it may be a bummer, but what I can't get my head round is that a film with an all-black cast is - for that very reason - being hailed as promoting  "diversity".  They must be using a different dictionary to me!