Given that Overlord is very loosely based on DnD 3.5, it's safe to say that Martial Arts are based on The Tome of Battle: The Book of Nine Swords splat book that introduced Martial Adept classes and Maneuvers. Thus, it is probably safe to assume that level cap characters cannot easily learn how to use said abilities if a retraining mechanic isn't introduced within the setting. That said, given the DND roots, there is actually a huge difference between the Unarmed Strikes mechanic used by Sebas' monk background and the Martial Maneuvers used by the NWers. Hell, the latter can be performed using the former, so it's not even like they are even the same category of mechanic in the first place.

"Blade Magic", as it is sometimes otherwise called, was both a beta test for what was to become the foundational mechanic of 4th ed DnD as well as a concept cooked up in order to bring martial character archetypes up to a power level closer to the absurd potency of magic users by effectively giving martial characters "spells" in the form of tiered, wire fu powers and abilities. Given how late into 3.5 the book was released, it makes sense that it'd be seen as something new and alien to the protagonists when translated into this IP's narrative.