The article is clear on only one point, but it's not an explanation for his winning. That point is that if he loses $500K, he only has to pay $400K of it. I'm pretty sure that if you bet $100K a hand, and stop whenever you hit the first of +$500K or -$500K, and get a $100K rebate if you hit the lower number ... well, the whole plan would seem to have a positive expected value. And perhaps the same would be true even…
> It was the dealer’s turn. He drew a 10, so the two cards he was showing totaled 15. Johnson called the game—in essence, betting that the dealer’s down card was a seven or higher, which would push his hand over 21. This was a good bet: since all face cards are worth 10, the deck holds more high cards than low. When the dealer turned over the house’s down card, it was a 10, busting him. Johnson won all four hands.
Pretty sure that by "betting that the dealer’s down card" it means that this was the logic behind his betting on the four hands, not that there was a seperate bet on it.
"Calling the hand" most likely refers to the kind-of-tradition of cheering on what you want ("come on facecard... COME ON FACECARD!!")