It was 17 years between the first potatoes arriving in Japan and the end of Sengoku era.
This article states that potatoes were not grown as food in Japan until early 1700s - nearly a century after the end of Sengoku era, and probably even more after this story (which iirc is set in some specific year), with them being produced on a large scale only by late 1800s when military settlers were sent to strengthen Japanese claim to Hokkaido in face of Russia, and western varieties being imported and grown for general populace in early 1900s
Thus, this stew was either put there by the author mistakenly, or the translator mistook a general word for a tuber for a specific word for a potato.
which is honestly an understandable mistake and I only care because I am a nitpicky bastard.
Thanks for translating this great series!