This specific smuggling/laundering strategy - mixing raw resources together to disguise origin - is a proven, profitable, relatively low-risk kleptocratic strategy.
This is what Russia does with coal in the so-called Luhansk & Donetsk People's Republics.
Look through ISW's summaries in early June, and the "rubles institutionalization" pattern you see in Kherson is the formation of another currency-exploitation zone.
Pensions are important for other reasons (it's financing terrorism) but fit here too.
If you really look at the grain theft problem in historical context, what really jumps out at you is that it's not "theft" - this is a somewhat clumsy term - as much as it is a very specific form of kleptocratic exploitation. And it goes back years.