When I do this, I’m supinated. The thumb heel (“thenar eminence”) is pretty fat. It sits taller than the pinky heel (“hypothenar eminence”). When I rest these two spots on the strings, the forearm bones are tilted. This is similar to the way Mike Stern does downward pickslanting.
Granted, this is not a huge amount of supination. It’s probably the practical minimum. But that was sort of the point of this little experiment, i.e. to show you how to do a very flat movement that’s almost entirely deviation.
When you work on this, a sloppy strum that approximates the shape of the movement would actually be more correct in terms of feel than trying to hit each not super accurately via stringhopping. So don’t be afraid to just wing it.
Filming also helps to see if you’re really making the movement.