I thought that as well.No sound deadening either.All the self build vlogs I've been following on Youtube insulate and vapour barrier all round.To be honest, I thought that too when I saw the photos!
It never feels cold at the back though and the only time we have had a problem was recently in Spain when we had our back to the very strong cold wind and we just hung up a blanket to stop the draught through the centre of the door which insulation would not have helped.