The contradiction comes into play only because οf a previous argument. Daniel said that we have seen enough over the years to infer that determinism is true, and claims that we have not fully explored the possibilities enough to accurately infer anything. When it is said quantum mechanics proves a random event, and the reply is that we just have not found the cause yet, then the determinist is going against the fundamental logic behind his strongest argument. For one argument the determinist says that we have seen enough to infer, but in a response to free will evidence, they simply say that we have not found it yet and that to infer that this is a random event is completely ludicrous. These are clearly contradicting and should not be considered because they negate themselves. The second problem with saying, "We just have not found the cause yet," is that it deems the theory οf determinism no longer empirical. For a theory to be legitimate, it must possess the ability to be proven wrong. By saying there is a cause, but we just do not know what it is, you rule out any chance οf proving it wrong, and therefore is no longer a valid theory.
All in all, determinism has a very strong foundation that you could base a sound belief on, but it does have its holes. There is real compelling evidence for determinism, and if you believe all οf the evidence then the holes seem very minor. Considering the evidence it seems that determinism could, certainly, be true. But before we close the debate, the free will theory should also be explained.