This sounds like pseudo science. Yes, it’s very common that people get stomach aches with milk because a lot of people are lactose intolerant and proteins generally take longer to digest. For some people the combination of being bloated and feeling full at the same time is very uncomfortable. It has nothing to do with hot or cold food. this whole discussion is completely moot for a lot of northern white people. A lot of them have a gene mutation that allows them to digest milk like any other protein.