The point is, plant matter is not geologically sequestrated in the present world. The oceans are oxygenated and circulation is pretty strong, and permafrost is decreasing, not increasing. Hence, any photosynthesized carbon material will be returned to the atmosphere as CO2 relatively quickly.

The only way to sequester carbon is to make something like limestone (Ca/Mg + CO2 + H2O -> (Ca/Mg)CO3) and that requires calcium or magnesium ions. There are some sources of those ions, but not that many. Another option is production of something like carbon fiber material, or even better, blocks of diamond.