Game theory says cartels can be unstable due to cheating, but the key factor is whether the cheating is detectable. With a commodity such as oil, cheating is hard to detect, so there are incentives to cheat. OPEC members cheat frequently; the members keep the cartel in place anyway, but much of the time, the cartel has little effect. With an employer cartel, cheating is obvious - the employer almost always finds out where the employee is going.
What's impressive about this alleged cartel is the audacity to keep it up, given the number of people who must have known about it: recruiters, hiring managers, HR folks, etc. There had to be 100 people at each company who knew it was in place, and nobody ratted for five years.