I am an RN and I think that a SLEEVELESS sack to put a baby in is about the dumbest thing I ever heard of. Hands and arms get just as cold as feet and legs especially in very young babies who have a poorly developed circulatory system and very little subcutaneous fat to help keep them warm.
Buntings in days of yore covered everything but the babies face and if they turned their head, they could suffocate. Where will it all end? Let the parents decide based on good sense (seemingly lacking here) and what their OWN physician advises.