the guy who tweeted about the song has deleted the tweet
as far as I know only Carrie's voice will be on the song with MWS playing piano so it could be seen as a Carrie song
Mainstream radio have been very good to Carrie, and her relationship with them is pretty important. If she is publicly debuting a new song at the Global Citizen concert, I think it would a be a significant courtesy for radio to have it simultaneously (if not ahead of the public performance). I realize they may be concerned about leaks - but I think that that may prove the wiser course than not giving it to radio for a couple of days (if indeed that is the plan), especially as members of the public are certain to record any new song and put it on the internet within a hour or so.