It's a very 'Guardiola' way of working.
Cancelo was out-of-form after a disappointing World Cup, started bitching about his younger replacement... and soon got dispatched.
This "already signed for another club" stuff is a stretch. Not like he'd injure himself on the bench.
I take it as: Sanchez was told he's not starting against Arsenal, and threw a strop. RDZ told him he shouldn't travel...