His explanation is kind of the high-fidelity version of the first day of digital electronics class. Digital computation relies on voltage levels that are high or low. The voltage level of the output of a step is not directly related to the voltage of the input, just whether that voltage falls in the low range or the high range. This fact prevents inexact voltage from compounding over time. Maybe you're using 3.3v logic; one step's input could be 3.2v and that would still be a high, and the output, if high, might be 3.5v. Non-digital systems don't inherently have that constant resetting of levels back to discrete values like digital systems do.