Returns of first fast serves are usually done around upper chest high, so a volley grip and technique can be either a pure block or a slight underspin, to a heavy chop.
First serves moving at you around 120+ just don't allow a sub 5.0 player to do more than barely block it back, with no spin intended one way or another. You just don't have the time to decide and take any kind of backswing.
When you reach 4.5, you have the time.
A top player facing Milo's or Roddick's first flats would almost always use a conti grip, to block or volleytechnique the returns. He would have NO time to take any kind of swing, or accurate grip switch.