Zariel may have had something to do with the testing of Gondian tech (varient of the infernal engine), but I otherwise agree that Zariel wasn't directly involved with the Absolute.
There's an interaction between Karlach and the Steel Watch, followed by something Karlach says once you find a certain person's parents in Act III that leads me to believe that the reason a certain person 'gave' another person to Zariel, may have been to test an engine design.