Yes, the old addresses will be compromised. That's fine. The point is that nobody can tell that you aren't actually using the same keys to generate new addresses anymore.
Although I would argue that even this doesn't have much value. It's not a big problem that people know "there exists an insider at OpenAI". There are plenty of employees there that shield you from being discovered.
In fact it would be so difficult to find this person among them, assuming the most basic opsec, that I'm highly skeptical they actually fired anyone. I would sooner assume this is just an announcement designed to discourage the behavior, since no specifics are provided.
Nonetheless, you can just be a pre-existing OpenAI employee. As long as you take basic precautions, they (as in, OpenAI), are not going to be able to find out it was you.
tell me you don’t understand crypto without telling me you don’t understand crypto.