Look, I don't get to control how people classify political parties. That is how SPD has always been defined. Centrist parties have always had a variety of status quo-esque ideologies. Hindenburg was a monarchist and a member of the SPD, because monarchy was the recently departed political system and he wanted a return to the status quo. Liberals were also a part of SPD, and there are a variety of samey progressive liberal ideologies that seek to entrench capitalism while providing social benefits to the people. That is how a liberal center-left party can exist.

And for the record, I'm an anarcho-syndicalist.