I would say that leather in its natural form is flammable, but due to chemical treatments it is possible to make it inflammable. One thing to keep in mind is that some firefighter PPE is made of leather, boots and helmets, I have never seen a helmet catch fire, and mine has served me well. I am not sure is FDNY still issues leather helmets, they are quite a bit heavier than the new plastic ones but there is more tradation behind the leather. There are also numerous tests that have been done with leather treatments in the automotive and airline industry to do with leather flammability and the vapors it produces when ignited.
