Not sure how much we can take that as being indicative of the future though.
There's very few acts that have ever headlined Download that would even be considered suitable to top the Pyramid Stage, so there's much less opportunity for there to be an overlap. Metallica, GnR, AC/DC, Muse, maybe MCR or Iron Maiden would be about the entire list.
My gut feeling is that Glastonbury haven't got Green Day this year, as I think Download will have paid for some form of exclusivity - but if the Download announcement doesn't mention that, would have to think they're a possibility.