Yeah, I'd wager the code is 9X% in "C" as OldRover knew in advance the game would have to be ported to Windows PC and to Sega consoles given the Kickstarter goals were met, so any PCE ASM speedup function planning/usage would be limited if at all. Code portability was very important here.
The Sega ports have been outsourced to another team, last I heard, and Rover decided to handle the Windows PC port himself despite the team originally thinking that should be outsourced also.