Because they never get hot, it makes it less likely they were caused by it unless you invent a whole story to include it.
To burn skin you don't need a high temperature, 40 degrees maybe a touch more tha'ts all. Then it's just a case of how long you are in contact with the source of the heat.
So saying the Aga does not get hot helps Bamber I think as it means Nevil was lying against the Aga for several hours at least.