Are Matt Damon and Ben Affleck geniuses, or did they have to do extensive research for the movie?

Research of what?

It’s rumoured William Goldman did uncredited script rewrites.

Their first draft was absolutely nothing like what this movie turned out to be. It started as a thriller. I suspect there were a ton of rewrites by Hollywood ghostwriters to turn it into what it ended up being. I'm sure they helped since they got credit, though.

They are not geniuses—they are merely gifted and talented people who work hard.

I would argue that discipline is more important than talent.