Some translations use the word "responsible" rather than "within our power" which makes a huge difference. I wonder what is the better translation.
I'm an amateur here, but I agree with Mary Beard, I find the common reading of "the dichotomy of control" (by the way - who coined that?) to be trite and so obvious as to be meaningless. If I cry and wail because it rains on my wedding day, I'm just an idiot, not a non-Stoic.
If you want to test if a statement is philosophical, you just need to imagine the opposite argument. What philosopher would ever argue that we should get upset if it rains on our wedding day?
I've been reading a lot of Hellenistic philosophy after initially been drawn into Stoicism.
My reading is that Epictetus is delineating selfhood (an inventory what we are and what we are not) and the extent to which we can cultivate virtue. Anything beyond a very narrowly defined self is "none of our affair" and, in fact, "inferior".
Epictetus "universalises" virtue in a similar way Paul universalised salvation, but he does so at a cost.
“If you should meet a person in tears”, he taught, “be careful lest the impression move you to believe that their circumstances are truly bad.”
To me, Epictetus disdains the world as indifferent and teaches his students to do so. I can't see it any other way when I read the text.
I wrote in detail on the point here:
The argument for Stoicism not being politically quietist, is circumstantial, not explicit, and so a matter of interpretation.
The "Stoic opposition" (a modern concept) doesn't necessitate Stoicism being an activist philosophy. I'm making an educated guess here, but this is perhaps more related to sociological aspects of Stoicism - aristocratic young men with a philosophic disposition meeting and talking with their similarly-minded peers. They would have despised (and feared) the emperors, their freedmen henchmen and the Praetorian Guard.
The popularity of philosophy schools would mean that these men met and talked politics and (republican) history. Philosophical schools were probably instrumental in ferment in the same way coffee houses, guilds and taverns have been in the modern world.
These men simply had Stoicism in common, but likely motivated by their own self-preservation.