Seats 21 DEF on that aircraft are not exit row seats. I have never seen passengers on those seats briefed by flight crew about exit row seating. Seat guru does not identify them as exit row seats neither:
https://www.seatguru.com/airlines/Un...767_300_ER.php
One way to make sure, when booking an exit row seat, a warning page comes up. Did OP get the warning page?