I wondered that too when I came on this site and saw people advocated it. All my life I have thought it was inadvisable to give puppies cold food and have always given them warm food. But as regards frozen kongs , it takes them a while to get it out, and in comes out in small bits, so it's rather like us licking a lolly! I still feed their meals at room temperature, but the frozen Kong when leaving them alone is a brilliant idea.
Lesley and Dylan