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.