They literally went "its a surprise tool that will help us later" so they can make up some random op shit on the spot that makes no sense in the context of the story (a videogame) and you ate it up.
This is no different to just making stuff up (they did for other abilities in this fight anyway) they just announced it beforehand which is 100 times worse cause that means the author didnt write himself into a corner he purposefully chose to go there lmfaoo.
What are you smoking. Definition of ass-pull is it came from nowhere. So it's still an ass-pull when the writer mentioned it before? Do you know how stupid what you just said sounds? Even then, the writer went to so much trouble of building this up. An arc showing how the MC got it. Repeating over and over how other players haven't found it, that everyone finds katana skills cool. The MC indicating the want to test this all out.
And the rest of the skills are pretty expected tropes for a MMO fiction. Like a tank swap skill, mental debuff skills bind skills, etc. Most importantly, Freya already stated long before she wanted to be a healer/tank, the writer needed to make sure she showed that. If anything, the writing has been really tidy.
Seems to me more like you have the barest of reading comprehension, tried to get smart, got called out for it, then tried to back out of the corner you wrote yourself into in pretty much the same inept and illogical way.