I'm very unsure if this date is really the definitive one. Nasty is out today March 10 and May 12 is 2 months from now. I mean, she has been promoting Nasty for almost 2 months, I'm not sure she will just move to a new single in less than 1 month time.
Well Summer starts in June and Pixie has said her album is coming out this Spring so this is probably the official release date. And I doubt Metro reports something without a source.