If I trust Wikipedia, even today the term "Erdapfel" is used to refer to topinambour in some regions: "Mancherorts in Baden wird Topinambur als Erdapfel bezeichnet."
https://de.wikipedia.org/wiki/Topinambur#:~:text=Mancherorts....
Topinambour would have been available in the middle ages. Confusion with potato by the modern reader is understandable, as a topinambour does look similar to a potato, and some German dialects (e.g. Swiss dialects) name potatoes "Erdapfel".