No, it's a fact - pros and cons. Tesla has no obligation to provide stock to employees - and so employees/unions get to decide what is the better potential option for them - and I suppose it's a risk for employees to unionize if they will no longer then be getting stock; unless a union somehow can force Tesla to give X stock to unionized employees, I don't know all nor have I thought through all possibilities - so there's possibly nuance missing but Elon's tweet itself isn't a threat.

Threat's the wrong word but purposefully misused, even if it causes fear or rather it will cause a fear of a sense of loss of value - because if employees value owning/gaining Tesla stock over whatever else they potentially could negotiate through a union, then there's certainly a fear of potential loss - but Elon pointing out that possibility isn't a threat itself.