Interesting how even in their very modern approach (compared to the hacking directly on Atari 800s their process is contrasted with) there seems to be no mention of source control, unless I missed it. This seems to point toward there being none:

Crucial to a team approach to software development is the ability to break programs up into separate modules and share access to them among the entire team. The UNIX operating system made this easy although a little more concurrency control in the various editors would have avoided one or two minidisas- ters. Often three people would be working on related parts of a game, passing files back and forth for advice or criticism while a fourth was compiling and testing the results of the others’ changes.

These days, I get antsy just thinking about any little personal-only project above a couple hundred lines not being in source control.