Jizz and jerk,
There is nothing to say you cannot start a duty knowing you will go into discretion. The specific wording is that you "cannot be rostered a duty knowing that discretion will be required to complete the said duty".
Subtle difference here.
With the case in point, should you arrive at the aircraft an hour late and the Captain has taken all the necessary preliminary steps (ie consulted crew, assessed previous rest etc), the Captain may decide to continue to the destination with the use of discretion should he think it appropriate.
The rostering department (god bless their little souls) cannot anticipate every little contingency like a bus driver with a toppled gyro. (sic)