Yes, I think "German portmanteau word" is redundant.
Reminds me of the German philosopher who toiled for decades on his three-volume masterwork. But it was hopeless, because he died just after completing volume two -- and all the verbs were in the last volume.
Well, I think that it could still be used in a broad and general sense, seeing that it just means blending two (sometimes incongruous) words together, but I’m by no means an expert on this matter (especially as it concerns German!).
A different thought: I wonder if the term portmanteau word makes sense in the context of German, given that concatenation is a standard mechanism for word formation, and thus doesn't represent anything unusual or noteworthy.