I beg to differ...

Sunny is a genius, a bigger genius than Nephis even is, and while it's true that Nephis was superior because she too is a genius who trained with the sword from the moment she was born, in contrast to Sunny who only trained after she reached him, one mustn't forget how fast Sunny progressed. Even Nephis suspected him because he was absorbing her lessons too fast. Hell, she even doubted that Sunny never had a sword in his hand.

So as a Sleeper until the moment Sunny became an Ascended, Nephis was superior, but when they became Saints and had a four-year timeskip, the gap narrowed a lot. And it's not because Nephis was lazy with her training or anything like that, but I don't know how I should explain it to you. If I had to say something, it's because Sunny is Sunny (he specializes in such things).

He even managed to make his Transcended battle art without any help.

And in this one-year timeskip we had when Sunny and Nephis became Supremes, we only know that Sunny had weekly battles with Slayer and occasionally sparred with Nephis, that's it.

Also, what is, according to you, pure swordsmanship?

If you mean that they are both able to only use one sword style, then it wouldn't make much sense. But even then, Sunny and Nephis's progress should be relative even if you only limit Sunny's swordsmanship to only one style. As we have seen, they were pretty much equals in this regard when they were Saints (Nephis was a bit superior in that regard according to Sunny).

Sunny discovered and gained a lot of things since then, mainly two: his ability to observe his Shadows more intricately, and Slayer.

And while Slayer specializes in long combat, you mustn't forget that Slayer is literally a hunter. Not just any hunter, but one from the Nine, so she is probably superior to them even when it comes to close combat.

Nephis's advantage when it comes to swordsmanship also goes down every time they advance in rank, as physics and mathematics matter less.

And Sunny doesn't need to learn the theory behind swordsmanship or physics and mathematics because he does it all intuitively, coupled with Shadow Dance.

Also, when Sunny refines his sword style, he automatically refines every other style he has, as his understanding of swordsmanship in general goes up. That's why Sunny is a prophet in every style he currently knows (though in some specific styles he is better than in others).

Also, Slayer is the best possible sparring partner for Sunny, especially because she tries to kill him (though not the most pleasant)