As someone who lived through the "Raven Symone as Elphaba" scare of the mid-2000's, I feel like this might be fake about Ariana Grande in the show.
AS for Frankie Grande, not a fan of him at all! Didn't like him on Big Brother, nothing. I don't like him. However, I will agree with the previous poster who said, its not like Wicked needs to "stunt-cast". I mean, celebrities have gone into the show (Rue McCanahan, Joey Mcintyre, Jo Ann Worley to name a few), and they didn't announce it full out like Chicago, or Cabaret.