That argument is bullshit, and I'll tell you why.

An employer can't fire people for unionizing, but there's no law that requires them to accept a union's negotiating demands... Or prevents them from bringing in scabs if the union chooses to strike without pay.

The existence of a union by itself doesn't do anything.

The only power that a union actually has is not showing up to work. And when the union doesn't show up to work, the employer is free to hire someone else to do the work. It's wild to comparing people not showing up to work because an employment agreement hasn't been reached to 'being held hostage'.