Their comment is supposing that the energy costs of hauling the tree out of the forest and slicing it into boards / mulch / whatever (given our current logging

Their comment is supposing that the energy costs of hauling the tree out of the forest and slicing it into boards / mulch / whatever (given our current logging technology) emits more carbon (from fuel usage and electricity generation) than the tree sequestered during it's lifetime. This probably not true, considering that for many decades steam powered logging equipment burned wood before coal became the dominant fuel source (and thus able to cut down and process a tree with less energy than we got from a single tree), although many mills were powered by flowing water. It's the same notion that "PV panels emit an equivalent of 50g of CO2 per kW/h (assuming a 25 year service life)". Even though the panel doesn't emit anything during operation, mining and manufacturing are still heavily reliant on fossil fuels. The tree itself sequesters carbon, but the carbon output from using the tree is what is produced burning it + getting it ready to burn.