Not to dismiss his achievement, but, why do people always think programming in assembly is ultra hard? This was just how we wrote games back in the day. I still know people using Assembly for a wide range of development, it's not particularly harder than any other programming. A bit tedious, yes, but with FASM and Fresh it's a lot better.
Noclip did a documentary[1] on RCT and successor games, and tried to interview Chris. The response passed through his representative is stated in the 2nd to last segment (The Legacy of RCT at 30m55) in the video.