I doubt that there was a "german" word in Old English.

There are 2 "german" words. One refers to what were previously called Teutonic people, from the name they used themselves, which was later changed to the name used in Latin by Caesar for them.

The second "german" word, which is also spelled "germane" comes from a completely unrelated Latin word which refers to siblings who share both parents, whence "hermano" in Spanish.

The second sense "germane" seems older in English. Perhaps it already existed as a Latin borrowing in Old English.

Otherwise, perhaps the author has used "Germanstuff" because no word for Titans existed in Old English and "titanium" was discovered and named by a German, i.e. by Klaproth, from whom a lot of chemical element names have come.