More food for thought.
Maybe there is some confusion about holding the rod with the thumb on top grip vs how one thinks they actually cast with that grip.
I've noticed that some fly casters hold the grip that way but when they actually cast, they rotate the rod hand so that the side of the fly reel faces forward on the cast. This places the thumb more to inside of the cork as the rod stroke goes forward. Then this becomes the palm out grip.
Here is a photo from Mel Krieger's book, The Essence of Fly Casting
. From left to right, they are the thumb on top grip, the V grip, and the palm out grip. As you can see, as you rotate the reel out, the casting grip changes.
As Mel Krieger's description indicates, the grip is how you hold your rod as you make the casting stroke.
As you watch the caster below, you will see that he rotates the rod so that the reel is to the side and he uses the palm out grip.