Water from a water heater should not be eaten or drunk. It's a health hazard.
But he is right about water that has been heated at some point freezing sooner. That doesn't remove all the junk in the water though. Don't drink water that has been inside of a water heater.
An instant hot is okay, those tanks are stainless steel, but not your common tank type water heater.
A water heater is designed with a "dip tube" that puts the new incoming cold at the bottom of the tank. Heat rises, so the hot water if pulled out of the very top of the tank.
You can reverse the order, but you won't have as much stored hot water ready to use.