I think most people (myself included) hold the phone with the bottom resting in their palm (and often further supported by their pinky/little finger).

Do you hold your phone from the top, with the bottom ending unsupported and hanging out? And if so, why?

Obvious downsides: if you loosen your grip then the phone will fall to the ground; you can’t reach the on screen keyboard.

How are you holding your phone? I'm typing this comment one-handed using a soft keyboard on the bottom of my phone. My thumb actually cannot reach the top of the screen without shifting the phone to a different grip.

GP is not saying that the top is easier to reach than the bottom. Reaching to the very bottom of the display one-handed is still straining, compared to the Home button iPhones.