Exactly; powerlessness is a big source of stress / anxiety / etc, but when you truly accept that you can't for example change other people or things outside of yourself will never be exactly the way you want them to be, then you'll be a lot more at peace.
To phrase it negatively, it's a kind of selfishness / indifference. But it's not "I don't care", but more "it is what it is".
The difficult part is to also be careful as to not become one of those YouTube "stoics" who just stay in "I don't care" state for everything. We can't control other people, but we can try to influence them towards a better path if needed.
I see some people accept with serenity the things that cannot be changed, but then have no courage to change the things which should be changed because they have no wisdom to distinguish the one from the other.
Spinoza has a much deeper and more effective approach to this, I found.