I did not say that it was all that stoicism is. I am not even claiming that it is especially central to stoicism. But it is undeniably a part of stoicism, as any fair reading of those two quotes indicates. Yes one of the central beliefs of stoicism is that we should inculcate within ourselves an ability to withstand any change in our external circumstances. But it is a complex philosophical system. It is perfectly possible for stoicism to admit both that belief, and the view that I'm ascribing to it.