I mean, Nothing but Thieves did 2 nights at Wembley Arena last year and drew a healthy crowd to their set at Glasto a few months back. So I'd say they're in the ballpark for a smaller end festival like this one.
If it's true they've been roped in to be a UK exclusive, that's quite interesting going too.
When I went to Truck in 2019, they were subbing Wolf Alice and drew a very decent crowd for it, many of whom knew and moshed along. Even if I left 2/3rds of the way through to see Public Service Broadcasting.