You really don't have an option if you're on a macbook - the keyboard rests a good sixish inches behind the front of the machine.

He means that your wrists shouldn't be touching anything. They should be hovering a couple of centimetres above the flat area in front of the keyboard.

Am I supposed to hold my hands in the air for hours straight? Because I can't do that, or at least it's far, far from feeling comfortable.

Yes you are... doing otherwise leads to carpal tunnel syndrome