Culture
They didn't even try to share culture with them, and culture is what explains their techniques
IF someone learned sword techniques, I think it would've been a small group of the numbers and the gifters and possibly Kyoshiro could have been the one to introduce them to samurai techniques. He would have led a special group.
Oda may have stayed away from an idea like this because he already has the Flying Six Fellows (Tobi Roppo).