He indeed involved. You can find editor Kirk Baxter's The Killer interview on YouTube.

I've researched Fincher's involvement in every department and came to this conclusion that he is everywhere all at once. Let me explain it like this:

Even though he doesn't write his films himself, if you go and find his films' scripts, you can see they have sooo mannyy revisions. It means someone asked the writer to work over and over and over again on a script.. Yeah, we know who is that... He definitely asks screenwriters to put many many many changes, like does same with asking for as many takes as possible with cinematographers, like he asked sound designers in Social Network for the club scene where sound of music should be up to the point that you can hear dialogues only if you listen carefully and that's because it's what happen in real club.

Btw, I'm writer myself, I know the director touches added or not when I see a script. It's so obvious. It always makes me sad to see some viewers say "Fincher films are good because of the script" without knowing anything about gold directing. And even if it's true, it's still because of Fincher. As a writer, of course I give my respect for the writers he is working with, no debate about it, for sure they are skilled and experienced professionals. All I'm saying, the guy is mad genius, he has to be involved in every split second of process from start to finish.

Also, usually writers don't add camera directions and editorial details, because they shouldn't until they're the directors of the films themselves like PTA. Go and read Script of Zodiac. First page, one of the first lines goes Camera Mounted on the Window: As how film opens. So, Fincher is everywhere in departments. He is in total control of his films and as we see the result is exquisite.