I don’t see that as “threatening workers”, but as a reasonable communication between employer and employees and a proper counterbalance against union claims that the workers will be better off if they unionize.
“We give our workers stock options now. No other automaker working under a union contract does so.” is something that I don't think anyone could reasonably find as threatening (even I don't think the first one is either, some may).
As an employee, I benefit from more information from both sides of the issue rather than having the union organizers be able to communicate unfettered and the company communications be restricted from pointing out any possible or foreseeable downsides.
Part of why this is not a simple "both sides" issue is because we're talking about one side that exists and one side that might one day exist. The non-extant union can't weigh in on company statements or make any statements of their own.
So these rules exist to help ensure the workers are not unduly influenced by the company when they consider making that union finally exist. I don't see a problem with this.
But if they're paying the workers part in cash and part in options, and the union organizers are pitching that they'll raise their wages, I think it's fair for the company to point out that that is likely to result in moving some of the comp from options into cash.
The UAW and representatives can certainly make statements today. Here's one: https://uaw.org/statement-uaw-vice-president-cindy-estrada-d...
On what possible basis could Musk be making a fully good-faith statement that he has good reason to believe it is impossible for a union to negotiate on such grounds? Obviously he is free to stick on that point in actual negotiations, but again he can't in any kind of good faith negotiate in public with an entity that doesn't exist yet.
As for UAW, that's kind of getting into the detailed specifics of this instance. Maybe there's more room to argue for looser regulations when there are extant industry unions? But that would also seem to push things more in favor of larger, more encompassing unions rather than smaller more specific ones, which I imagine most people in favor of less regulation wouldn't be fond of either.
And the entire point is that the union forming could be bad for workers in various ways, so if I was a worker I would want to be made aware of what could possibly go wrong before I joined the effort to unionize and no longer had a choice in the matter after the union is fully formed and going strong.
Musk has a significant say in whether employees continue to receive options, and I can’t see why the union would ask to take that away, given that options are presumably very popular with their members.
Saying that life will get worse with a union is one thing. Saying that it will get worse because I will make it worse is another.