I'm actually on Musk's side on this
I'm a union person. I've been a rep and stood alongside people in arguments with management. To a fair degree I agree with him. If the people in a company feel they don't need a union and the company is treating people fairly, then crack on. The only time I've been needed as a rep is when one side or the other, have gone militant and mutual respect has broken down. The lords and peasants thing is a bad analogy, though. Even with unions present, people rise within good companies.
Unions from outside the company forcing Tesla (and their workers) to have a union when they don't want one, I disagree with. That a union can force other people outside a company, to cripple that company from outside... that's not right IMHO.
In this case, however, there does seem to be a union inside Tesla which is having issues and negotiations aren't working, which have triggered sympathy strikes. So I feel like I don't have the full facts here.
"Maverick: The Success Story Behind the World's Most Unusual Workplace" was a very interesting read on engagement and although they went full employee ownership, it proves that there are different ways of engaging employees and company operations.