Isn't Wikimedia a non profit? Who exactly are the employers on the other side negotiating with the union here? They're higher on the hierarchical structure, but they're not actually stakeholders the same as there are in private for-profit companies.
The union is negotiating with management, i.e. those with hiring and firing authority who direct the work of others. Same as a for profit.
Uh, non-profits absolutely may have unionized employees:
I... wonder if you don't know what a non-profit corporation is or how they work?
Some of the largest unions in the world are for teachers, police, and government employees. I’m not sure why you would think a for-profit company is uniquely subject to its staff unionizing?