I'm not a lawyer, but I would also guess they need to "flip" these folks against OpenAI and get them to cooperate in the lawsuit against the actual folks with big pockets. I think they're essentially alleging a conspiracy by OpenAI and they need as many examples as possible to make the case that this was a pattern and standard practice, not just one or two idiots acting on their own.
So if I'm a former Apple employee and I get one of these scary letters, I'm asking my attorney if I could get out of a lawsuit by sharing any information I have about any potential OpenAI shady practices.
That's overly dramatic.
At this point, the assumption would be that they are a non-party witness.
So, beyond not destroying any potential evidence, you might as well tell them to shove it.
You shouldn't ever willingly give up information to a plaintiff if it could implicate you. If the information exists, it's going to come out in discovery. Admitting to theft of trade secrets is probably not going to help you, it's not like the cops offering you immunity for turning state's witness.
You talk to a lawyer and do what they say, not what Apple demands of you. No one but a judge can demand anything of you.