Determinism does not say that opinions or understandings cannot be changed by exposure to other ideas or understandings. Does it?
Well, I could be wrong about this, and my understanding of determinism says that the outcome of any interaction is the result of a causal chain starting from the beginning of time all the way to present. There is no "changing" anything as any "change" that may happen was pre-determined to happen anyway. Under determinism your attempt to "change an opinion or understanding" in another person is no different than a "rock potentially bumping into another rock". The outcome of such an "attempt to provide input to a system" is pre-determined anyway. That persons present state and even final state is pre-determined according to the various "bumps and nudges" they have recieved and will recieve throughout their life.
This is what I believe is meant by "there is no purpose in debating unless free will is true". Under determinism, people will end up being whatever they are "destined" to end up being. Debating becomes pointless as there is no chance to alter the system in any meaningful way. In fact, your "choosing to debate or not" was pre-determined. In determinism NOTHING is causeless.
As always, this is just my understanding of things. I could be wrong at any point. If so, let me know.
"False ideas never die; only their supporters eventually snuff it." - Hervé This