I only glanced at the code in ANIM.CPP but for a C++ program from 1997 it looks kinda nice, right? The methods are all short, with some descriptive comments at the top of each one. Inputs and outputs described. Nice index at the top of the file. I’ve seen worse!
Yeah, I was looking at infantry.cpp, and the quality is actually very good for 90s game code. The documentation is good, it's decently well-formatted, and there are assertions. Nice counterexample to the "all successful games have terrible code" conventional wisdom.
That's a modern wisdom from the last 15-ish years when people started to have good enough internet connections that you can get away with publishing a dozens gigabytes patch on launch day.
In ye olde times, it was prohibitively expensive (or in the case of console ROM cartridges, impossible) to distribute patches, so projects were usually planned with plenty of buffer time and plenty human testers. These days it's rush to not collide with the releases of other AAA studios, and human paid-for testers have been replaced by free (or, sometimes, paying) early-access players.