Your argument doesn't even legally work. Mens rea is only a requirement for criminal responsibility. It protects you from going to jail, nothing more. It prevents a public prosecutor from suing you to get you into jail. It does not apply to cases brought by counterparties for financial damage caused. If you destroy a bridge by driving a truck into it (you know, failing to consider the height, the typical fuckup), you're paying for the bridge. If you set a building on fire by forgetting to turn off a gas heater in the garage, you pay. If you hit a pedestrian on the street, you are responsible for the medical care needed. Etc. Intentional? Not intentional? Doesn't matter for the damages.
Second reason it doesn't work is that Mens Rea does not fully apply here. These companies' AIs have hacked, not (fully) UNintentionally either. The companies did ask those AIs to do stuff (apparently the request was literally to hack, just only within the environment), and were watching what the AIs did to some extent ... and let it happen. They then failed to take adequate measures to prevent reoccurence. As any "conspiracy to commit ..." case will teach you, this constitutes partial or passive Mens Rea, and that is certainly enough to convict people. To give an extreme example: if you give a night club bounder 100k and tell him to "take care" of a named problem, and he kills that person, you are an accessory to murder. Including in the case that he misunderstood you. Even if HE didn't intend to kill the person, it's still murder, and you are still an accessory to murder.
That some excuse was needed to let billionaires demonstrate hacking for free? Obviously. And whether it works, according to law? I mean one might hope that they would at least have gone through the trouble of changing the law, but since that's enormously complicated and fraught with unintended side effects, they haven't even bothered.
The other excuse is national security. You see, Big Bad China is threatening and that means we must let the billionaires do whatever they want (this was brought up as an argument in the copyright cases)
Of course, one might ask, if we do that to beat China ... even if it works, what's the point? We'll have become authoritarian at that point and the only thing that that changes is the face of the people holding all the power. Of course which face holds all the power is really important to billionnaires ...