The J stroke works well on the outrigger canoes and normal canoes as a steering stroke.
The idea is to push the rear end of the canoe away from you.There is also a slight delay before you notice the change in direction at the front.
I don't see it being of any use on a SUP, as we tend to pull the nose around to catch waves more quickly. Using more of a sweep stroke, would be more suitable for changing direction.
The draw stroke works well from leaning out and pulling water under the board at an angle of say 45 degrees from your hips to the nose of the board.
I suppose that is why we are called "sweepers".

Thats my 5 cents worth.