I have a similar technique with my right hand thumb and I can trace the genesis from when I was playing in a church band and dropped my pick and had to execute some arpeggios and the thumb had to fill the gap since I couldn’t bend down to pick it up without calling a lot of attention to myself.
It wasn’t very clean at first but serviceable but found myself doing it more and more.
Now I routinely use my right hand thumb for all kinds of things as part of my regular technique.
He’s definitely utilizing it to run fast scale patterns in a unique way. Nice!