Because when Glastonbury’s plans have fallen through in the past, ‘exclusives’ and other shows haven’t stopped them, e.g… Metallica in 2014 who were ‘exclusive to Sonisphere’ and The Who in 2015 who cancelled a gig in Paris to do Glastonbury.
I’m not saying I think Green Day are playing, but I wouldn’t write it off completely based on them being ‘exclusive’ elsewhere, as it’s happened before.