There's a lot of intertwining between both warrior traditions and occult practices, globally. Not having yet watched the link, did they mention the legend of the Tengu, and how they are connected with the Ninja clans? That's a good story, and it's not surprising that there's the idea of these dangerous trickster mountain spirits taking an especially deep interest in the Ninja clans...enough to share wisdom and skills with them.
There's a lot of intertwining between both warrior traditions and occult practices, globally. Not having yet watched the link, did they mention the legend of the Tengu, and how they are connected with the Ninja clans? That's a good story, and it's not surprising that there's the idea of these dangerous trickster mountain spirits taking an especially deep interest in the Ninja clans...enough to share wisdom and skills with them.