I think many people wish "union busting" in the past meant sending out an email arguing a position and asking people not to unionize.
And if we are to immediately dismiss any argument Musk makes against unionization because he is the business owner, as some have argued, why aren't we also obligated to dismiss any argument the employees make in support of a union? I think all claims should be evaluated regardless.
I think the interactions between unions and businesses are complicated. It may certainly be that Musk is making false or unreasonable claims in his arguments against unionization, but to simplify this as just "unions good, arguing against this union bad" is a little much.