Sure, but if each session has a unique key then these need to be managed and stored and unauthorized access to these leaves tracks. So all that had to be 'compromised' is a single universally applicable key. Again, the question stands: session based encryption can be scalable and efficient. Why aren't they using it?
The exploit here isn’t a leaked encryption key. It’s pretty likely that they are already using a unique key per conversation. The raw CoT eventually reaches the model, and you can convince the model to share it with you.