Hey Cryo,
Yeah, I'm with you here. From what I remember, the story goes something like this:
Takamatsu Sensei was a quite sickly and weak child, often bullied by his school-mates, and nicknamed "Cry Baby" (which had not yet gained street cred as a name, as Johnny Depp was many years off immortalising the name...). As a result, his father sent him to his Uncles school, which (publically) taught Shinden Fudo Ryu at the age of about 9 in order to "toughen" him up for a planned military career. For the first year or so, the young Takamatsu was not taught any techniques, and was basically used as a punching bag fo rhte other students. He was hit, kicked, and thrown around until he bled, occasionally knocked unconscious, revived, and then thrown again.
Eventually he began to be taught the techniques of Shinden Fudo Ryu, being granted Menkyo Kaiden in just a few short years. During this time, it was revealed to him that Shinden Fudo Ryu was not the only system taught there, but Gyokko Ryu, Koto Ryu, and Togakure Ryu as well. As he began to be taught these arts, he had a preference for Koto Ryu, as it had a focus on conditioning and hitting, but found Ninjutsu (Togakure Ryu) far less appealing. I feel that this is due to the way he was introduced to the school, with skills such as Shiho Ten Chi Tobi and Shoten no Jutsu were first presented as more "games" than training drills. After all, Togakure Ryu's tiajutsu curriculum is not anywhere near as large as schools such as Gyokko's or Koto's.
So I do feel that it is a mis-interpretation, and is based more on the fact that Takamatsu was a child when he was first shown the aspects of the art, but none of that means it is "based" on kid's games.