well, I have not learnt butterfly yet, so I cant speak from personal experience.
Just watched the direction of the arms movement when the swimmers exerted force on the water, and that seems to be directed more downward then in freestyle on avarage.
Can you really come so high out of the water without any arm force as in full stroke?
I must say, the single arm butterfly looks very similar to freestyle regarding the arm path.
Indeed, not much pressing down here.