Sure, great question! I do talk about this in the video somewhere, but I’m using the G major scale to walk between these C chord waypoints, because we’re approaching this lesson from the aspect of playing a C chord in a song in the key of G. The 16th fret is an F#, which is contained in the G scale. In the lesson, I point out that if that bothers you, you’re welcome to play the 15th fret and it would be fine to do so when in the key of G as well.
Essentially, I don’t want you to think of this lesson as being in the key of C (though it works great for that!), but rather a song in the key of G with a bunch of C chord measures in a row.
There are some songs that have the freedom to hang on certain chords for an indiscriminate amount of time, songs like Freeborn Man and others, where the band hangs on a certain chord till the singer signals the change when they come back in. This exercise mimics something like that, though anytime you’re over a C chord for 1, 2, or more measures there are pieces that you can use.