Good reminder that the word "union" is overloaded and doesn't always mean what you might assume.
People who join a union like this don't have union representation or union contracts with companies. They rely on individual members to take action in various forms within there own companies. In theory they could call for a strike and ask everyone who is a member to strike from their respective companies, but in practice it's more about raising awareness and making noise in hopes of driving change.
So while technically it's a union, it's not comparable to what most people think of as a union in the United States.