But the article you quoted says nothing of that. The arguments in the German media revolved more around the German corporations attacking Müntefering and the Left Wing happily adopting it and blowing it up, like this IG Metall magazine. And of course the unions and the SPD have a history of cooperation on each other, but you assume wrong if you link them like you do. It's about as wrong as assuming the Arbeitgeberverband is a subdivision of the CDU. They have common interests, but naturally the IG Metall is much more socialist than the SPD, and since the party tries to adopt this New Left image it's a growing pain in the ass for them.
Page down to near the bottom of the list of "DGB-Mitglieder unter den Abgeordneten der SPD-Fraktion" and you will find: "Müntefering, Franz IG Metall"