The film series is skipping over the second and third novels in the Millennium book series and going straight to The Girl In The Spider’s Web; by doing this, the films can effectively create a sense of distance between Fincher’s film without actually disregarding it.
This is ultimately helped by the fact that The Girl In The Spider’s Web is not the story that immediately follows The Girl With The Dragon Tattoo; instead it is the fourth.
The Girl In The Spider’s Web is technically a sequel to the 2011 film directed by David Fincher and exists within an established continuity.
That said, the overhaul of the cast certainly makes it feel like a de facto reboot.