Interesting! I had never heard of Peel Ply before; but I did note the same effect when I was epoxying the scarf joint on my gunwhales. I used a thick piece of plastic "drop cloth" from the box store to protect the angle iron I used to align the scarf joint. Some epoxy squeezed out and after it dried, the plastic "drop cloth" peeled away cleanly and left a very smooth and even surface on the cured epoxy squeeze out around the joint. Same principle at work, I suppose.
I did a little more research on Peel Ply and it seems it is only recommended for use on simple, flat surfaces because it doesn't necessarily conform well to complex shapes and can leave "folds" in the epoxy as it sets up.