Basically all courts agree that the rights of an individual to find employments to support their own needs and their family trumps a company's desire to beat its competitors.
If Apple really were worried about losing secrets, they could have paid for garden leave, e.g. pay employees who leave for 6 months to just stay at home.
If your goal is to protect the company it seems easier to enforce than a makeshift cartel agreement with other CEOs
Also, wasn't the agreement about recruiting being taboo, but its ok to hire cold applicants?