It's not rigged. Arch0wl makes his own stepfiles, so he knows the song he's making. He got Marvelous and Perfects, so it wasn't a AAAA. (The highest rank in Stepmania)
I mean, editing the timings is easy as hell, but with the amount of stepfiles he's made, it probably just comes naturally. I do it myself, not that accurately, but at that speed.