The case at issue on Friday dates to 2017. At the time, Mr. Ortiz was known to be involved in an effort to unionize the company’s plant in Fremont, Calif., which included distributing union material and pressing for pro-union legislation, when he posted screenshots of his co-workers who were anti-union on a private Facebook page.

When a company investigator asked Mr. Ortiz where the screenshots came from, Mr. Ortiz said he didn’t remember, though a colleague had sent him the pictures after getting them from an internal human resources site. Mr. Ortiz later admitted that his response was a lie, and the company claimed it fired him over the false statement.

The labor board concluded that Mr. Ortiz had been fired because he was engaged in union activity, not because he had lied. The circuit court agreed that “substantial evidence” pointed to the conclusion that “union animus motivated the complaint, the investigation and the decision to terminate Ortiz.”

Mr. Musk’s post from May 2018 about stock options said there was nothing stopping Tesla’s employees at the plant from voting for a union, adding, “But why pay union dues & give up stock options for nothing?”

Tesla argued that the statement was a straightforward prediction based on Mr. Musk’s understanding that other members of the United Automobile Workers union did not receive stock options, and that his benign intent became clear in subsequent posts in the same thread.