I thought the guy was fired for lying during an investigation, not for union activity? The employee had every right to share union information so I do not understand why he / she would lie about that in the first place. Anyhow, an appeal is almost certain in this case.
Among other things, Tesla security guards repeatedly ordered union organizers to stop leafletting in Tesla's parking lots and fired one union organizer for allegedly lying during a company investigation. Elon Musk was also dinged for a tweet that suggested employees would no longer receive stock options if they voted to form a union.
