It pays to pay attention to the specific wording of his post. He really doesn't want his workers to unionize, and the reason he dared UAW to hold the vote is because he knows that because of historical reasons UAW is really unpopular in Fremont.

In short, Fremont used to be a GM plant and have their own union. When this union was absorbed into UAW, it was fairly contentious and many in Fremont believed that UAW didn't care about auto workers in California. Then very shortly after, UAW negotiated with GM about downsizing, and the end result was that the Fremont plant was shut down. This was seen in Fremont as a total betrayal. Soon after, Tesla bought the plant.

So UAW really isn't liked, and any vote about organizing under them would be a landslide against. A different union might be a lot more successful.