What is wrong with staging? I don't even consider staging until it is time to commit, at which point it is a little area to gather the exact changes you need.
From what I can tell about JJ's split command, it seems like it does the same thing except you're working backwards to split out changes into another commit.
Both seems fine, I just don't see the complexity with the staging area (unless you're a beginner who's been told to only use the CLI at all costs).
Jujutsu's authors have pointed out that a simpler model (here, and with first-class merge conflicts) helps them move much quicker. There are many weird and exceptional states with Git that just don't occur, so new features are more straightforward to add.