They can't sequester any more carbon than was in the feed stock. And when you're trying to sell a biodegradable thing (which I think is laudable), this would have a minimal net effect on carbon for a material that is readily disposed and biodegrades within a few weeks. The benefits of reducing plastic waste, landfill, and pollution are obvious. The carbon benefits are less clear. Paper packaging from pulp waste comes directly from a photosynthetic resource, and may be found to be equivalent materials. So the argument for mushrooms as an intermediate seems like it needs more explanation -- to me anyway. Obviously, a bioplastic that never degrades is a far better carbon sink than a material that degrades readily, but then you are generating waste that must be stored somewhere indefinitely. Not a problem for a building material, but obviously not great for packaging. But I don't see how a building material made from mushrooms could ever sequester more carbon than, you know, wood.