As much as I'd like to think that it's true, I wouldn't consider this legitimate yet. There's no way Arcade Fire and Foo Fighters would be on the same day unless we're going to get some really obscure headliner on one of the other days or something.
Crossing my fingers though! I missed them back in November
if the site is legit --- what it most likely means is that Arcade Fire is expected to headline at some point during the festival - but haven't released the exact day of the performance. Therefore they just listed the first night of the festival.