I guess it's like how in this world you can just google how to do techniques that were state secrets in the past. Like metallurgy or glass works. people died keeping monopoly on these things. Martial arts are no longer the deciding factor to war in a world of guns, missiles and drones, so solders can learn as much as they want. And training techniques have been refined for hundreds of years. Though, to be fair, humans have had military training for hundreds of years but there is just rarely surviving documents.
https://mangadex.org/chapter/911483/10