We have no idea how it would even be possible to make these horizontal adjustments with the typing machines of the time. I agree with the other comment here that the authors haven't used a typewriter, or at least in enough detail to know that on many if not all of them, holding down the spacebar will advance the carriage by half a character (and releasing it, in normal use, advances the other half.)
The authors are the famous Kernighan and Brailsford (!). They spent their life on stuff like this, and know him personally very well. They are talking about spacing much finer than half-character.