How is it possible for a real materialiist to say anything other than "well, the theist has no more ability to freely choose to believe in theism than I have to freely choose to not, so it's pointless to argue"
Because a change in the environment can cause a change in beliefs about the world. If the theist were forever determined to choose theism in spite of what facts are presented to him, then you might have a point. But changes to a person's understanding of the facts can lead to changes in a person's beliefs, even if those changes are determined. Thus the motive to present arguments, on the chance that doing so might change the theist's mind. I fail to see how this is at all controversial. At the moment I encounter the theist, perhaps his prior thoughts constrained him to being a theist, but when I enter the scene, it's not a predetermined conclusion that his beliefs and choices will remain the same. Because I am an animal that feels emotions, I am motivated to do things which supposedly will further my interests. I'm here on this forum half as a proof of concept, that the experience will not change my views, and half curiosity about what leads others to the theistic conclusion. I am open to the possibility of being changed, but that will only occur if I am presented the right reasons for changing. This is all still perfectly determined. I consider it in my interests to engage, regardless of whether I was determined to come to that conclusion or not. I cannot predetermine the outcome simply by assuming that it is predetermined. I must go through the process to determine the outcome.
But even if the environment changes, and the belief stays the same, then essentially "it has gone through to the keeper".
Consider a flood, you realise that your house is about to be flooded and you put sand bags to protect the house, yet the flood still breaks through the sand bags. It is simply the forces of physics at play here.
Much like when a theist or an atheist hears arguments for and against God, under determinism, neither party can rationally affirm the evidences and conclude what they believe in because of the validity of the argument.