My best guess is that it's counterfeit and the guy "aged" it to try to hide that fact.
I wouldn't be so sure. The article says that this same guy has been doing this for years with different amounts. If it were counterfeit, I guarantee that the Feds would know about it and the guy would be in jail somewhere. You don't take a suitcase full of Monopoly money to the Fed and ask for a check.
No, I'd bet it's all real money.
My completely uneducated guess? I bet that a couple of decades ago he found a crashed drug-smuggling plane out in the desert and managed to bring home a couple of duffel bags full of cash. Woohoo! But.. what would you do with it? I might do exactly what he did. Play it cool and lay low. Exchange small amounts as often as you can without raising suspicion. Obviously he got greedy with what was left (5.2M) and decided to try to exchange it all in one shot. Tsk tsk.