This means that manager unions do not gain the traditional protections from the law that other unions do. But - if a manager union and employer come to some agreement despite this fact, it has legal weight and the union can sue if the employer breaks the contract (and vice versa).
Managers, at least at the higher level like being discussed here, arguably do not need as many protections in order to be able to collectively bargain: Apple is likely to be much more concerned (and thus much more willing to negotiate) about 20 "Director of X" employees leaving than 20 engineers.
Managers, at least at the higher level like being discussed here, arguably do not need as many protections in order to be able to collectively bargain: Apple is likely to be much more concerned (and thus much more willing to negotiate) about 20 "Director of X" employees leaving than 20 engineers.