I think the implication is that China had previously imported much of their reserves and is now reselling at an opportunistic time. The argument for sanctions would be similar to the argument against price-gouging or scalping.
Are they reselling? I don't think China is really exporting any grain at all in any major amounts. They can't satisfy domestic demand and prices are soaring. To then sanction a country for not selling to you is kind of bonkers in my mind.