It only applies to certain microcontrollers, but he provides example Arduino code showing how handy this can be for battery-powered projects. The usual way to measure VCC is simple, ...

Sometimes errors, typos for example, are reported in multiple locations but the correction is elsewhere. ... That means I’d expect a source code for ab Arduino project to be like 100 kB at most.

Official Arduino Example Tutorials: The Arduino computer software is loaded with tons of examples for you to use in your own code or to run through as a learning experience.