It's most likely just a concept. There's no real - world event where Harry Potter as a character actually kills fanfic. It's more of an idea about how the official Harry Potter material could potentially impact fan - made stories.
It's a concept. While the Harry Potter franchise has had an impact on fanfic, it's not in a literal 'killing' sense. The official books, movies, etc. can overshadow or make some fanfic less interesting, but it's not a real event. Fanfic is a creative space that exists alongside the official works, and while changes in the official story can influence it, it's not a case of one literally destroying the other.
Just a concept. The idea of 'harry potter kills fanfic' is a bit of a hyperbolic way to think about the relationship between the official Harry Potter series and fan - created fiction. There are no real actions within the Harry Potter universe that can literally kill fanfic. However, things like new canon information or changes in the overall perception of the series can affect the popularity and creation of fanfic, but it's not an actual event of 'killing'.