It's just that under the assumptions implicit in materialist philosophies, there is no compelling reason to be good.
Maybe not compelling to
you but the existance of moral atheists shows that it is compelling to
some. The reason is that it is the right thing to do, in the particular person's opinion, or that it benefits the most people, or that it will make someone hapy. The materialist philosophy's implicit assumptions do not
remove the already existing reasons for being good.
As such, the goodness of unknowing naturalists finds it's source purely in the cultural programming they received from a theistic source since past.
I'd need support for that assertion to accept it but it might be off topic and, admittedly, its not of much interest to me.