Since we don't know exactly what 'anbu' is in this context, it's difficult to determine the full significance. However, in general, fanfiction often tries to expand or re - imagine the original story. By saying 'Harry Potter is anbu fanfiction', it could mean that the fan wants to show how the world of Harry Potter can be changed or enhanced by incorporating this 'anbu' element. It might be a creative way to introduce new story arcs, character relationships, or magical elements that are not present in the original books or movies. For instance, if 'anbu' has a unique fighting style, this could be introduced into the wizard duels in the Harry Potter fanfiction.
The significance might lie in the combination of two different concepts. Harry Potter is a well - known and beloved franchise, and 'anbu' (whatever it specifically refers to in this context) is being added to create a new and unique story. It could be a way for the fanfiction writer to explore new themes, like loyalty, secrecy, or power in a different way by bringing in 'anbu' - related ideas. For example, if 'anbu' has a certain code of conduct, it could be compared or contrasted with the moral code in the Harry Potter series.
I'm not sure about the significance without more details. It could be that for the author, 'anbu' represents a set of values or characteristics that they want to introduce into the Harry Potter world to make it more interesting or different.