When an agent prefers one of two possible acts, the discussion asks whether that preference requires a determinant and, if so, what determines the determinant. Requiring a further preference at every stage threatens an infinite regress.
The argument aims to show that human volition is not an absolutely independent power of origination. But analyzing the grounds of a preference does not by itself remove the action's voluntary attribution to the person. The nature of choice and the creation of the capacity to choose are distinct issues.
This is not a recommendation to abandon effort because destiny exists. It is an attempt to understand voluntary action without assigning independent creative sovereignty to a creature.