I would say when people use the word 'anime' as an adjective they're specifically referring to the extremity that anime and its tropes are associated with.
For example, in a traditional medieval fantasy setting it would probably be fair to settle for a bunch of younger men with exceptional sword fighting skills compared to regular humans with the same level of experience. Anime would stereotypically ramp such a concept up to eleven and make it a story about a bunch of teenagers with beyond exceptional agility, speed, and skill compared to anyone alive despite these kids being ostensibly unexceptional by the world's standards.
Similarly, huge weaponry, extreme hair colours, ornate outfits that don't hamper agility, huge fauxlosophical diatribes by either side, idealised looks/body types for everyone, and the stakes of the story at some point or another ascending to planetary/multiversal significance all fit into the aesthetic/extreme feeling that most anime and manga tries to evoke.
So I think their remarks regarding your character is just that the concept of a very young person being extremely strong and capable of wielding a massive, usually impractical weapon is evocative of the anime aesthetic.
Edit: It should be noted that yes, anime aesthetic and superhero/comic book aesthetic are very similar. I would argue the core difference between anime and superhero/comic book style is that while anime's extremity and aesthetic appear to be a core part of the world the characters inhabit (the world is simply this over the top), with superheroes, they are an extreme, over-the-top facet of an otherwise grounded, mundane reality. Anime style is about relishing in the extremity itself, superheroism is about relishing in the juxtaposition of the extraordinary against the ordinary.