I think the operative words in that article are 'above ground' - Plants kept in above ground containers need to be hardy to at least two zones below whatever your zone is. That means a plant would have to be hardy to Zone 4a in your area to be safe in a container above ground for the winter. So sinking them into the ground is a good option. I'm not sure what the difference is between "in the ground" and "in the pond that is in the ground".
The water in the ground and the water in the pond are both going to freeze solid and the temperature will be lower than 32* because of the ambient temperature of the air. But an above ground water garden or planter box will be less insulated so the plants will have to withstand colder temperatures.
