As I think someone else mentioned, he did headline (and curate) a sold out APE last year. Previous recent headliners of that include Stormzy, The Strokes and Jungle, all of whom could headline the other stage so I don't think it's totally unreasonable. That said, he might be better off going the Little Simz route and doing an afternoon slot on the pyramid which would probably get him more exposure.
Arguably thats not really headlining - the Glade headliners are on at 1.30-3. Camelphat FBS and someone else i think it was last year
Sarah Story is tiny in comparison, she's just a radio 1 dj that gets a few festival slots