When I was talking about neutral, I meant that in undisturbed forest over a very long time, as trees grow, they take carbon dioxide out of the atmosphere, and as they die or burn or otherwise are eliminated, they put that carbon dioxide back into the atmosphere.
So forest fires have always happened. They have always been part of the natural cycle of forests, but to the extent that changing climate is causing fires to be either larger or more frequent or cover a larger percentage of the forest, then that can cause a net addition of carbon to the atmosphere over and above this neutral balance of trees growing and dying and burning.
It's the difference between the kind of equilibrium state and a state that is changing as the climate is changing.