One thing I also think you need to consider: your editor isn't picking up on or strengthening some of the passive tenses in that section and is allowing too many redundancies in there. It's a small sample that you provided, but if you're really wanting to go down the quality road, then you'd need another pass on your chapters to get rid of things like "he realised", "it seemed", "the one and only", passive phrasing etc. Also review sentence structure and flow. Those sentences are all kind of slow and samey, but I get the feeling that the character is meant to be shocked or surprised by this information.
So an edited version would look more like:
Nathan had practiced his special move for hours. He still hadnât seen any progress. Even after the fight with the thugs, he still had room for improvement. Especially his Shadowstepâthe one special move heâd personally discovered.
But now he wasnât the only one using it.
This takes everything you've written and makes it pop. It removes words that weren't doing any heavy lifting, strengthens your verbs. Adds a break that highlights the importance of that last sentence etc. I'd also say that one thing that's missing from this is your character's feelings about it. Is he angry? Is he surprised? Is he tired? It's hard to say. He's been practicing for hours, seems to have found out someone else can use his special move that he's trained so hard to master, and yet feels very little emotion about any of it.
It's a tiny sample, but this is the kind of things I'd pick up on as an editor if I was editing it with quality over quantity in mind. Hope this helps a little with strengthening your work! Honestly, there's nothing overly wrong with the sample you provided, it's readable and most Tapas readers will be absolutely fine with it, but there are things that will push it up to the next level as it were.