Do capacitors provide ESD protection even if they are already charged?
But what if the &quot;protective capacitor&quot; is already fully charged when
the ESD event occurs?

It still provides protection; if more charge is put into the capacitor from an ESD surge then, the capacitor charges up more and the voltage increases. Of course, if the voltage rises above the maximum rating of the capacitor then it will likely damage the capacitor. And, if the voltage rises too high it might become too high for the circuit it is intended to protect.
But, basically, it is a great way of protecting against ESD surges. Not so great with indirect lightning but good for ESD level energies.