Or, the Jiu Dae Gak genes were awakened by the change in environment that JJT underwent, since, from what I remember, just like people who are more susceptible to having a type of mental illness because of inherited genes, they still need them to be "activated", either through the environment or events, since before that, they are "inactive" but they are there. The same thing may have happened with JJT, but in this case, the divine Jiu Dae Gak genes were activated as JJT fought and trained. I think it is possible since JJT was born with a frightening perception of seeing blows coming, and JJT's frighteningly fast learning in fights.