The company firing all previous employees is a risk of a strike. Striking is a negotiating tactic. Tesla's response is hostile negotiation but it appears that the employees agree that what they have is better than whatever else they could get.
If you're being threatened of being fired for even negotiating ("striking"), would you continue to "negotiate"? Might be that they don't agree they have it better than elsewhere, but the alternative is being fired, so they'll just be fine with the situation as-is.
Threats are negotiation. The union threatened to strike. The company let the union know what their response would be.
Question is, what "participating" actually refers to. One could make the case that striking is just a part of normal participation in the union, while one could also make the opposite case of course. In the end, seems it'd be up to the courts to decide.
That means that all other unions will refuse to work with that company, and this goes for both suppliers and customers of the blockaded company. No more transportation services, buying parts, getting service people to show up when your equipment is broken, delivering your goods to customers, etc.
So, being blockaded is a risk which should be known going in.
https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/em....
But the employer can then still refuse to take the employee back. In that case the employer owes the employee 6-32 months pay in damages, and the employment contract becomes legally terminated.
Source: The Swedish law on employment protection, paragraph 39:
"Where an employer refuses to comply with a judgement by which a court has annulled a dismissal or redundancy or has declared a fixed-term contract to be of indefinite duration, the employment relationship shall be deemed to have been terminated. The employer shall pay damages to the employee for his refusal in accordance with the following provisions."
Fun fact: managers have their own union in Sweden as well! ("ledarna", literal translation: "leaders") And as the other commentator mentioned, you'd most probably lose your position as the company if you participate in bullying, and if your union finds out, you'll most likely get kicked out of the union as well.