Disproving an argument is not a prerequisite for exposing hypocrisy, these acts can be done separately. And if the other party doesn't address the accusation of hypocrisy there might not be a need for disproving the argument to them, as their position might be politically motivated and not aimed at resolving the issue in principle.

If Charles Manson had said murder was wrong, we might have doubted his sincerity but hardly the point of the actual statement.

in this case, it is more akin to Charles Manson condemning Shoko Asahara for cult activity, and dismissing his "then why are you doing it?" as "whataboutism".

It is important to know whether the condemning party applies the same standards to their own activities, because there's no possibility of resolving the issue at hand without both parties applying the same principles and standards on everyone, including themselves. And dismissing the significance of this knowledge as "whataboutism" is short-sighted, because it's the tool of establishing standards of morality, and it gives a hint to third parties about the nature of their neighbours involved in the dispute.

"There is a precedent and the precedent was followed" is the other sides argument.