Kun didn't invent the saberstaff he gained that knowledge via a holocron and reintroduced the weapon to lightsaber combat.
Still, his opponents didn't know it and needed to adapt to what was, at the time, an unconventional style. That's often been noted as one of his advantage.
But in Krayt's time, it's a very conventional weapon where later masters have added their own techniques and flourishes and such. Rather than Kun being the only saberstaff wielder around, Krayt's from an era where both Jedi and Sith use it.
Krayt had experience sure, but when Kun is calling Luke's own dueling skills 'quaint' we have an idea of just how much he himself knew about the art.
I'll note this was relatively early Luke who was still growing, and frankly, I'd say Kun was just fronting. I don't see him doing any better against Sidious.
Kun don't forget fought two of the best master swordsman of the era, both Form V practitioners and held his own in a power duel, even killing the more experienced of the two.
Krayt fought against the two next best people from his era. He killed both.
And note that Kun didn't so much outskill Vodo, as pound him until his staff broke.
It goes to show that he is the only one who genuinely mastered the Niman form, clearly adapted it to his own preferences and has kept the poster boy tag even after 4,000 years of other duelists.
I'm pretty sure a number of others have mastered Niman. Sidious mastered all the forms iirc, and taught Maul all of them as well.
And, well, Krayt. Krayt's genuinely mastered it and that's who he's facing, so not much of an advantage there ^^
Simply put Kun was a natural as a swordsman, the fact he mastered multiple uses of the lightsaber in the most adaptable form, is much more impressive when you realize he was practically self taught.
... he learned under Vodo, known to be the greatest lightsaber teacher of the era, for years. He's not like Nomi who largely started on her own.
And, frankly, mastering multiple forms is pretty darn common in the higher tiers. Nor is it necessary to be high tier- Dooku and Kenobi were extremely skilled while focusing on one form.
Kun is indeed quite a natural as a saber user, I'm just not seeing how that's supposed to make him better, especially when his foe is both quite talented and way more experienced.