Wikipedia[0] has this:
Stoics outlined what we have control over categories of our own action, thoughts and reaction. The opening paragraph of The Enchiridion states the categories as; "Things in our control are opinion, pursuit, desire, aversion, and, in a word, whatever are our own actions. Things not in our control are body, property, reputation, command, and, in a word, whatever are not our own actions.". These suggest a space that is within our own control.
Is that based on the self-help book approach?
[EDIT] not to say their physics & metaphysics were constant over time—Stoicism's period of serious activity spanned centuries, after all—and that much of their "lower level" philosophy wasn't very likely produced by working backward from their conclusions, which is common in philosophies that attempt to prove or support their ethical beliefs in such manner (see also: "natural law" or "natural rights")
[EDIT EDIT] and I'm not claiming that original approach is the superior one. It's just why there might be a difference in opinion over what's essential about Stoicism. The utility of a Stoic mindset may well be pretty unrelated to an underlying approach to the problem of evil, a statement about the nature of the divine, claims about what reality actually is, and so on.