Primavera has sold out the fastest ever after Chappell, Charli and Sabrina were announced as headliners. That doesn't mean they have to headline Glastonbury, though. They're very different festivals and selfishly I'd rather not see all three twice in the same month lmao. But I don't think any of them would be out of place at Glasto, especially Chappell and Charli. Anyway, if they end up as subs, there will be a huge crowd.
I'm guessing its because he still has his Ally Pally gig in February, and might not want to distract some of the excitement from that; something might get announced afterwards