How did David Fincher splice Brad Pitt into multiple frames?

Regarding whether "it's all editing", the answer is yes, and that's not just this splice. All of what you see when you see a finished movie in a theater is the result of editing.

In his Masterclass, David Mamet repeated a maxim regarding movies: "You make a movie three times. Once when you write it, once when you film it, and once when you edit it."

To actually answer the question, the frames that were spliced in were filmed at some point, and likely many frames or even feet were filmed. Then, during editing, a few frames were selected for each splice and edited into the desired spot in the film.

In terms of how he filmed it, they just picked a set and had Brad Pitt in costume and makeup and filmed some footage. They also could have easily spliced in footage filmed for totally different scenes.

When Fight Club was made, digital film editing using tools such as Avid was quite popular, and Fight Club was almost certainly edited digitally. This makes adding frames much easier than editing by cutting film. But splicing frames by actually cutting film isn't super difficult. Fight Club shows Tyler doing this in his spare time while working as a projectionist.