He often makes arguments like that if determinism were true, it would be impossible to change another person's mind, because, as he has so often said, people talking to each other would be like two television sets standing next to each other, each playing what's on the channel it's tuned in to without being affected in any way by the output of the other set.
It would seem, then, that what he means by "determinism" is not simply the position that the world is deterministic, but that each person is a closed deterministic system. ("Closed" meaning "not affected by anything external to it".)
One brand of the bizarre position that people are closed deterministic systems is the hypothetical position of "genetic determinism", which I doubt anybody holds, but that is often talked about and attacked by people who take the side of "nurture" in "nature vs nurture" debates.
In order for somebody's traits and behaviours to be 100% determined by their genes, they would, indeed, have to be a closed system, since to say that they are at all capable of interacting with elements of their environment is to say that things outside their genome can at least have some short term influence on their behaviour.
But of course, this isn't what any determinist means by "determinism". Determinism is the idea that the world is a (closed) deterministic system. Of course people can interact with each other and change each other's minds in a deterministic world. All it means is that it is determined that Paul will change Peter's mind. Or, if Paul learns about determinism and gets so confused by the concept that he concludes that there's no point in trying to change Peter's mind and therefore doesn't, then that's determined.
Whether a system is closed or not and whether a system is deterministic or not are two completely independent questions. I'm really not sure that Stefan Molyneux understands this. Does he?