Yes, you're being instructed to open the snowball. Also, the diagrams show small arrows. Those arrows are telling you which direction to press your seam allowances. IMHO, the quilt industry has a long way to go in giving pattern directions. Press open could mean anything.
I bought a pattern for a bag once, "Bow Tucks Purse" or something like that, read the entire set of directions three times, got a nice headache and no idea what they were talking about.