One considers the prospect of controlling what one can control.
This prospect is wildly different to engage when what you cannot control does not affect you, than when it does.
Furthermore, on the far end, it is a tautology. It is, of course, impossible to control what you cannot control. So, it does not limit the course of action at all.
Instead, it encourages you to do what you can, which differs from leaving some things you could do undone.
But, of course, one could do a great many things, and one cannot do all of those things.
So, given a great number of things you could control, you are admonished to control some possible subset of them.
Thus, the true value of this phrase is not in any meaningful management of what you can control, but rather a focus on ignoring that which you cannot.
You have so many things you could control, and you should focus quite hard on deciding which of those to control and doing so, that you have no attentional surplus to put towards that which you cannot control.
The question, then, is: Can you ignore that which you cannot control?
If something is going to happen to you, and you cannot control it, but you can react to it in some ways, then perhaps should pay some attention to it, even if you cannot control it.
This is why this advice hits different for someone impacted by that which they cannot control than for someone who is largely unimpacted by anything they cannot control.
You can tell, perhaps, that Marcus Aurelius was an emperor.
I'm being silly, of course. Nonetheless. Lots of things one cannot control. And sometimes they impact you.