For the example you shared, the paint is still fresh on project Vesta. Mining, crushing, transporting will have a non-neutral carbon impact which is not taken into account into their model afaik.

Overall our atmospheric models are not yet mature, which make carbon-trapping mechanisms from the atmostphere hard to gauge. Easier if we reduce or trap carbon emissions before it is spread into the atmosphere.

Please also consider we will eventually run out of fuel/oil/gas/coal; carbon dioxide savings is the long term strategy.