I think that the combination of shelter when stowed and heat from grow lighting when out the risk of the pitch becoming water logged and then freezing in a frost are eliminated. I haven't seen any heating system go in under the grass. A bigger question is how on earth drainage works. Does the grass pitch drain through the NFL pitch? Will the NFL pitch be stained and muddy after a rainy prem match?
When they installed the trays before they laid the top layers of sand and soil you could see what looked like pipes or hoses winding along the trays, rest assured there is undersoil heating. The pitch won't actually be stored away, it will always stay in situ in the open, it will only be moved under the south stand when the lower pitch is required for NFL or another event and it will be moved off as late as possible and moved back again as soon as possible, it will never be stored under the stand for any length of time at all.