I watched some of his videos. Basically he says ninja were never a group of people different and separated from the Samurai and the rest of the Japanese society. The distinction between a samurai and a ninja was just about occupation or role. That means that some of the samurai were involved in the shinobi role or occupation. And that the belief that both samurai and ninja were enemies is false. And as to Ninjutsu itself, it was never a fighting art or style according to his research. It was just a set of special skills used for espionage, sabotage, infiltration, breaking and entering, etc. And if there were ninja who were very good at fighting, it was because of their previous training in fighting arts.