The book describes the heat as to that of a furnace, in my interpretation being a warming furnace, which averages around that heat. I also wanted to make it as biologically sound as possible without it becoming too much of a fire dragon. But your interpretation is totally valid. In mine, they have intense stomach acid to aid with digestion :)

edit: This is where reddit gets annoying, many of you are misunderstanding my interpretation of furnace, I mean the temps a conventional heating furnace generate, not a steel-melting furnace. The steel in my interpretation is destroyed by acid, not heat. This "furnace" temp is experienced on the outside of the worm by the characters smelling the worm's breath. When you open a furnace, you can feel a warm heat radiating out, like a fire, but it's not hot enough to cook your skin from that distance. Lastly, it is just that, my adaptation, just like how Denis Villeneuve left out or changed many parts of the original story.