That's ridiculous. It undermines the whole principle of war crimes. For there to be a war, one place has to have been invaded by another. Since invasion by aggression is already illegal, following your logic would have us throw away all remaining war crimes, if the aggressor has already broken the law and the defender can now do whatever they want.

The Geneva Convention doesn't have a get out clause if you're the one being invaded. Civilian targets and infrastructure are off limits, invader and defender alike, because civilians often have little control over the war they're caught up in and don't deserve to be killed, starved, conscripted, detained, or otherwise denied rights because of the decision of their leaders.

This should be simple stuff. The world has gone insane.