It would be a pretty big c*ck up by Muse if they said no to DL thinking Glasto or R&L would put in an offer, only to get no offer from either. They must have had something in the pipeline elsewhere to say no to DL. I just can't see any reason they would not play any UK festivals at all
Again, this is total nonsense. We can have an idea of who is booked by believing people that pass this info on every year.
Same way SZA Dua and Coldplay was known last year with the latter 2 being firm and the former being a last min addition. Same way it works every year!