I hate to be "that guy", but here I go.
Right off the bat, he makes the mistake of thinking that, if a chord needs an extra note to make it whatever jazzy chord you're looking for, you can just add it as a bass note.
This is not
quite true. C/A is not a C6 chord, it's an Am7 chord.
Yes, the two chords have the same four notes. But there's an important difference in which of those notes functions as the distinctly lowest root/bass note. It's not just a matter of equivalent inversions, like it would be if you played A-C-E-G vs. C-E-G-A in the right hand only. That low-pitched bass note is setting the stage for how the entire chord sounds, and how it functions in the overall harmony of the piece.
If the accordion is a "band in a box", then those single, left-hand notes are the bass player. If that bass player sees C6 and plays an A instead of a C, they're either making a mistake or taking it upon themselves to reharmonize the tune.
The Am/C he talks about as a second option is the more-correct way to play C6 using just the left hand.
I stopped watching after that, so maybe he gets into this matter later?