It's really odd that the burls wouldn't burn well but I think you're right. I may have seen similar effects while woodworking (wood can easily get burned by the friction that some tools make). Burl is considered a very hard wood and difficult to work with but they are very sought after. It's just amazing (I think) that they are porous but fire resistant. You would think that a more porous wood would be more apt to feed a fire. Maybe the pores are filled with something that the tobacco burns off? - just thinking out loud.