Maybe Andy forgot to mention it; it's been a while since I've read the whole series.
The code was C and lisp so it didn't really require any effort to port other than replacing the rendering pipeline. And we used the SGIs to pre-render every frame anyway, to precompute the polygon sort order. (The PS1 had no Z-buffer, so you were stuck sorting polygons at run-time if you didn't do something clever.)
So we already had the rendering pipeline ported. Obviously you couldn't save your game to the memory card, etc. -- some stuff didn't work. But the game was playable (albeit very frustrating with keyboard controls).
The code was C and lisp so it didn't really require any effort to port other than replacing the rendering pipeline. And we used the SGIs to pre-render every frame anyway, to precompute the polygon sort order. (The PS1 had no Z-buffer, so you were stuck sorting polygons at run-time if you didn't do something clever.)
So we already had the rendering pipeline ported. Obviously you couldn't save your game to the memory card, etc. -- some stuff didn't work. But the game was playable (albeit very frustrating with keyboard controls).