Storing them in sub zero conditions is fine as long as you take the proper precautions before using them.
From that bulletin:
It is recommended that Summer tires are stored indoors at temperatures above -7°C (20°F) when not in use. If the tires have been subjected to -7°C (20°F) or less, let them warm up in a heated space to at least 5°C (40°F) for 24 hours or more before being installed or driving a vehicle on which they are installed. Inflate the tires only after they have been warmed above 5°C (40°F). Do not place tires near heaters or heating devices used to warm the room where the tires are stored. Do not apply heat or blow heated air directly on the tires. Always inspect tires before use after being stored.
