If the sawdust and twigs was just going to be dumped to rot, then it would give off methane, so similar to food composting, burning that output can be GHG negative. Its not going to be a primary source of power, because solar and wind are so ridiculously cheap and scalable, but every little bit helps in the short term. Once fossil fuels are elimanted it'll make sense to do other things with them, rather than burn them.