Well, there's a bit more than ferns and ivy on the forest floor. Think of all the young trees. Think of everything the forest creatures eat (they don't each up to eat evergreens 10 feet off the ground). But the most important fuel for the fire are the rotting logs you mention. Fallen limbs and dead trees left standing (or fallen over). These are very dry and in some areas are several feet deep. It turns the forest into one very large fireplace. Kindling on the bottom, large logs on the top. It's perfect if you want a massive fire.
Speaking as someone who used to do a lot of backpacking I know that thick underbrush and dense sapling growth is not a characteristic of a mature forest. It IS a characteristic of a young forest or a cleared area that gets a lot of sunlight. I am not saying a mature forest is fireproof,...yes it has logs and leaves..I am just puzzled over this particular point which doesn't jibe with my own direct observations.