I watched that video (terrific performance of a great song), and thought to myself "geez, I wish my guitar sat so nicely on my right leg!
So I tried casual position without a strap - as I have so many times before without success.
But some thing was different this time. My guitar never felt solid in my lap before - always wanting to slide off my leg. But this time it felt secure.
I realized the weight of my right arm was being transmitted diagonally to my thigh - holding the guitar securely in place. (See arrow in photo below)
I must have tried this position dozens of times over the years, with multiple guitars, and never found the sweet spot. But now it seems obvious.
baffled shrug emojii