There was a thread a few years ago about different stages capacities. I think most of it was pulled from the festival license that states maximum capacities.
Probably changed now with the loss of camping
And as is so often the case, given that no bookie has been offering it as an active market for a couple weeks, it's based on entirely fictional odds rather than actual betting.
This is 100% a combination of a bookie (in this case Betway) looking for some free publicity and crap news outlets looking for clicks. Glastonbury and Taylor Swift each bring those things so why wouldn't they combine the two into one story?