The fact that he sent an email follow up to a phone conversations means that 1) he had already decided not to become part of the collusion and 2) he wanted to leave a paper trail which would be made public if the collusion was ever detected.

If they've had off the record phone communication about the subject, the decision to follow that up with an email is a calculated decision. If he had decided to participate in the recruitment embargo, it certainly would not have been acknowledged in an email for all the discovery reasons that make this a compelling PR piece.

On the flip side, Steve Jobs comes off as being completely ignorant that this is discoverable. I don't know why he'd include threats in an email like this.