> I'm not sure what this whole thing is even about

The choice to voluntarily recognize the union would remove an obstacle for its creation. Deciding not to, in that context, can be seen as choosing to leave the obstacle in place. That, in turn, can be seen as intended to prevent the union from forming. The union is primarily a means of increasing bargaining power for its members. Therefore, this decision by WMF can be seen as intentionally weakening their employees' bargaining power.