It doesn't matter that they're not musically in the same key. It's their physical position on the fretboard that is being compared to the physical position of the white keys on the piano
You're missing the point. He's not claiming that musically sliding a chord and playing the white keys are the same, he's saying physically they're the same. Both are physically natural and so could be chosen for physical rather than musical reasons. There are, of course, also musical reasons to select those options -- like you said, the white keys are C major -- but that is not necessary to motivate the choice in either case. If I didn't know that the white keys on a piano were C major, I'd still be likely to play them because they're physically convenient.
