I agree with everything you say, but Goodfellow was a manager. Managers can’t join unions in the United States.
...which effectively states that if a supervisor joins a union it's a no-op. The employer is not required to acknowledge their membership. In practice, unions specifically exclude managers for a bunch of obvious reasons related to their ability to bargain effectively on behalf of their members.
Your manager likely joined a union-ish entity open to supervisors. IIUC such entities are not protected by the NLRA and as such have few of the legal powers and protections that make a union a union. They’re basically affinity groups.
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.