No that is not true there is a process called pyrolisis that takes place in wood that is exposed to elevated temps over time. That process slowly lowers the ignition point of that wood. It can get to a point that in a chimney fire can ignite that wood. In addition that chimney has been deteriorating for all of those years as well. The mortar in the joints between the clay tiles erodes away allowing creosote to send through putting combustibles even closer.I should have taken a picture of the croc. It was so nice and round and smooth like the inside of a brake drum. Yes that diagram is exactly what I saw. At least they knew what they were doing when built.
Being the house it this old, it would have went up in smoke long ago, well before my time if something was wrong with that chimney. It is 37 feet from the top if chimney on roof to clean out in basement.
I am not saying anything is nessecarily wrong with your chimney or crock because I don't know I am not there looking at it. I am just telling you that problems are very common in these areas and assuming it is fine because nothing has happened yet is very dangerous.
Ok so with a wider heat sheild you will be in good shape.With heat shield you see in picture, top of stove pipe is 6.25 inches from bottom of mantle.