"The Kaga ikki, also known as The Peasants' Kingdom, was a theocratic feudal confederacy that emerged in Kaga Province (present-day southern Ishikawa Prefecture), Japan, during the late 15th and early 16th centuries. The Kaga ikki was a faction of the Ikkō-ikki, mobs of peasant farmers, monks, priests, and jizamurai (lesser nobility) that espoused belief in Jōdo Shinshū Buddhism. Though nominally under the authority of the head abbot of the Hongan-ji, the Monshu, the ikkō-ikki proved difficult to control."
https://en.wikipedia.org/wiki/Kaga_ikki

Besides, your quote is fake.
It's "the legendary Peasant Kingdom", with capital letters and definite article. One might say that the adjective legendary is not used in an appropriate way, as it not a legend but a hystorically existed faction.

"DEVELOPED IN COOPERATION WITH JAPAN HISTORY EXPERTS
We put a lot of efforts into the research for the game. Every part, , from the tiniest stones to culture and politics should be historically correct. Only some lacks of information in Tokyo's university libraries push us to historical limits and challenge our creativity."
Yes, it looks they understand something about early modern Japanese hystory - I wouldn't call the Sengoku ancient. It's contemporary to the Renaissence.