However, knowing that Split takes place in the same version of Philadelphia as Unbreakable gives new meaning to Kevin’s backstory.
The final scene in Split shows that David now knows about Kevin’s existence, after having caught a news report covering the story will have to address.
Elijah might be more willing to help David deal with Kevin, after having (presumably) spent the last seventeen years in an institution for the criminally insane, following the events of Unbreakable.
Similar to how Unbreakable unfolds as a comic book-inspired superhero origin story for the character David Dunn has now led the character and his domineering personalities to a very dark place.