I would say that an organized group of workers that calls itself a union and advocates on behalf of those workers for labor rights is a union, and I think most people and dictionaries would agree with me. If that's not what you meant by the word "union", I think the burden is on you to provide an alternate definition for this discussion.
For example: "an organization of workers formed to protect the rights and interests of its members" - https://www.merriam-webster.com/dictionary/union