(mu4e-mark-execute-all &optional NO-CONFIRMATION)
...
If NO-CONFIRMATION is non-nil, don’t ask user for confirmation.

(define-key mu4e-main-mode-map (kbd "x") (lambda() (mu4e-mark-execute-all t)))

(global-set-key (kbd "C-x C-x") (lambda() (mu4e-mark-execute-all t)))

Wrong type argument: commandp, (lambda nil (mu4e-mark-execute-all t))