@SSS26032002 if you read the earlier chapters, you'd realize there'd been soo many summoned people, made into slaves. And, we know there's a LOT of slave types, including sex slaves and battle slaves. So...some kingdom might've gotten the idea of making a shinobi unit from the knowledge of previously summoned slaves and had them breed.
It's not that some isekai idiot made a culture here, it's that powerful kingdoms been summoning isekai slaves and been stealing
their culture.
Also, don't think that tying them up bondage style is simply for kinks. The tortoise shell rope bondage technique is really quite the strong restraining technique...outside of it's frequent featuring in kinks. Our MC being in a martial arts family taught skills that can are for
killing would've had learned this technique as a way to completely restrain an opponent.