It sounds right in theory, but you have to consider the planting and transportation costs. Plants that grow for longer absorb more carbon, and the petrol cost associated with tractor tilling, planting, spraying pesticide and fertilizer are proportionally lower compared to the volume of carbon absorbed by growth. Claiming buried evergreen trees are carbon sinks ignores all of the carbon set-up costs associated with establishing the trees. I’d believe it if we made tannenbaum out of bamboo and algae, but I’d have to see more data to believe Christmas tree growth was a carbon sink.

Moving it from the tree farm to the city, store to home, home to waste collection, waste collection to landfill is another practical carbon release. Landfills are real-estate, also a limited resource. While material decays much more slowly, preserving carbon not the intention of landfills. Modern landfills do account for or encourage the release of decay gasses and burn them off or tap them for power. They release the carbon dioxide is slower than incinerators (the much more likely destination for used trees) but probably not on a slow enough scale to make a geological impact.

You are correct that tree growth is carbon neutral, and something additional must be done to prevent decay and sequester the carbon. Simply growing trees to maturity and then sequestering them as bio-char on-site is more likely to result in net sequestered carbon.