Although my experiences have been somewhat limited on CX Y, how it worked for me was that at the time when you purchase the tickets, the exit row seats are not available as DM (I'm talking about the real designated 'preferred' seats on row 59 and not on row 39, referring to the A330 here)
However once the booking has been paid for and you can manage it online, I have been able to select seats from the designated exit row, although not all of them were available.
Similar situation with PE seats as well where row 30 wasn't available at the time of booking but once booked, manage my booking allowed me to select 30A/30K.