If enough heat, oxygen and pressure is present EVERYTHING can burn/ corrode. So I don't think its about splitting hairs, its about the meaning of the spell and its wording. It is obvious only light sources can be ignited by the first spell and the other spell says flammable: and then I assume stuff like pitch, oil, tinder, wood and things like dry rushes and the like.
Some stuff isn't meant to burn: a moldy rotten thatch roof just won't ignite, unless the heat is enough to drive the moisture out before igniting it. Solid oak beams will scorch for a while, but without more fire sources it will peter out, leaving a 1' burn mark, unless, perhaps it is so dry that the fire can propagate itself.
Its a GM's discretion area. Some will be fine with it, others will be less enthusiastic.
My players have learned, though, that oak WILL burn when you splash it with lamp oil before using the ignite spell...