That could be nearly carbon zero (I don't know the contributions from soil and what mass remains as ashes, etc..), except when you consider the opportunity cost.

That burned tree, presumably healthy just prior, loses the ability to sequester any more carbon. A burned tree puts carbon in the atmosphere AND removes a carbon sequesterer simultaneously. That act is removing X amount of future sequestration until the tree would have naturally died and turned to soil. At that point most of that carbon would still be in solid form and not in the atmosphere.

I'm nitpicking at this point, but I lean toward burning wood as not carbon zero for the sake of opportunity cost.