used to, but in that case it is a much larger company, they wouldn’t write this letter for so many reasons, first being you cannot fire them based on who they vote for without opening your coffers to their lawyers.
From what is implied here, the guy probably only has 20-40 employees, unlikely but not impossible they are unionized (engineering firm?.
Most likely, the letter was a hoax. It was just nice for a while to think that you could tell employees that.