Trees are very approximately 20% carbon by weight. While the forest grows it removes carbon from the atmosphere. Once it is a mature forest then the rate of growth = the rate of decay on the forest floor as old trees die and new ones grow so no additional carbon is taken up by a mature old growth forest. When we talk about plantation forests however we harvest the wood and replant so carbon is continuously taken up. We use the harvested wood to make structures or furniture or pallets or in particle board which continue to store the carbon until they have completely rotted away to nothing or are completely combusted.

Growing a forest and leaving it in situ without harvest will not lock up as much carbon as plantation wood that is continuously in the growth phase and then harvested and used for long term structures, boats and furniture which can last for hundreds of years or even longer if properly cared for and kept dry and free of rot.

The article does not mention if the proposed forests are grown to old growth and left in situ or are plantation forests where a large percentage of the harvested wood is preserved in usage while new plantings take up more carbon.

I dont think the models of today accurately predict the amount of warming we are likely to see in any case but its important to make the distinction when we say that forests will not have a significant impact we need to state the assumptions behind the type of forests and how they will be used , managed and grown.