If you have a 20 gallon water tank, and you drain it, at the most you might have six ounces of water remaining. That will freeze and expand, but not even to 2 percent of the size of the tank. So no damage will be done.
YOU do NOT want to fill the storage tank with anti-freeze and I doubt you could use compressed air to get all the water out, a few ounces will remain.
If you filled the water tank with anti-freeze it will take probably hours to flush out all the anti-freeze in the spring of the following year.
I have followed those instructions for three winters now, and have had no problem. And although it stays colder longer in BC than in UT or ID, we do have a few months of below freezing weather to contend with each winter. And if not winterized correctly, all it would take is a week of below freezing weather to cause damage.
Bikendan answered the differences between flagstaff and rockwood quite well.
Have fun; it is NOT a hard job to do and takes maybe an hour at the most and that includes the time to drain everything and remove/install lines.