Romanda unswearing and reswearing the oaths and not dying doesn't tell us anything we didn't already know. And that is, that she hasn't reached her oath-bound lifespan allotment (whatever it is).

That tells us nothing about what might happen to Mesaana if she were to swear the Oaths. For one thing, don't know exactly how old Mesaana is - she could easily be 50 years older than Romanda, for example.

What Sidious is suggesting is that the Oath Rod does not allow channelers to live past a certain age, regardless of how old they are when the swear. If he's right, and Mesaana is already older than that "certain age", then she would die if she swore the oaths.

Romanda's unswearing and reswearing doesn't disprove Sidious' theory, since we already knew that she had not reached that "certain age" yet.

Do you see what I mean?