Perhaps it implies that the essence of Harry Potter's story in the canonical works is complete, and there's no need for more fictional fighting scenarios as seen in fanfiction. The official narrative has run its course, and fanfiction's fighting - related ideas are now redundant.
One reason could be that they feel the official Harry Potter story has a defined end, and the fighting in fanfiction is just over - done. They want to focus on the original tale's conclusion.
It's an opinion. Fanfiction is a very fluid and creative area. There's no definitive end to it. Even if a lot of fanfiction has been written, there are always new ways to approach the characters, settings, and themes of Harry Potter. It's not like there's a set number of stories that can be written, and just because someone feels it's 'done' doesn't make it so in the overall scope of the fanfiction world.
One characteristic is the creative use of magic in battles. Writers often come up with new spells or unique ways to use existing ones. For example, in some fanfictions, a character might combine two spells to create a more powerful effect during a fight.
In fanfiction, 'Harry Potter Master of Fighting' could be a concept where Harry Potter is depicted as having extraordinary combat skills. Maybe the author wants to explore a different side of Harry, one that is more focused on physical and magical combat mastery compared to the original story.
In my opinion, 'harry potter is done fanfiction' can be understood in different ways. On one hand, it could indicate that the fanfiction community has explored all the common themes and plotlines related to Harry Potter. For example, we've seen fanfictions about the characters' further adventures after the books ended, their relationships in different scenarios, and what - if stories. On the other hand, it might also mean that from a creative perspective, new fanfictions are not being produced as frequently or with as much enthusiasm as before. This could be due to the fact that the initial hype around the series has died down a bit, or that the most obvious and appealing ideas for fanfictions have already been written.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.