The objection uses expressions of inevitability and impossibility: if divine knowledge is true, the known act must occur. The answer does not allow divine knowledge to be mistaken. Instead, it clarifies that what is known is the person's voluntary doing or omitting of the act.
Certainty that an act will occur is not identical with the claim that the agent is forced against the will. The action is known precisely under its voluntary description.
The distinction is between the necessity that true knowledge not be false and a supposed necessity of coercion imposed on the agent.