If you have 20% downside protection in a nearly even odds game, you can figure out the optimal strategy. Also, he was betting $100k a hand, so he didn't have to play too many hands before deciding to stop as a winner or take his 20% loss rebate. (If you have the 20% loss protection, the optimal strategy is to bet big.)
For example, his downside protection was 20% of a loss of at least $500k. If he was at -400k, he had no reason to quit; he should keep playing until he was ahead or at -500k.
Also, casinos previously negotiated 20% loss rebates on a LIFETIME basis. He negotiated the 20% loss rebate on a PER-TRIP basic, correctly pointing out that if he was ahead at a casino, he had no reason to return if another casino also offered the rebate.
The lt;dr summary is: He didn't cheat or count cards. The 20% loss rebate combined with blackjack being a nearly even game gave him an edge.
You could do the same thing at craps. At craps, there's an obscure rule that lets you make a bet at true odds after establishing a point, sometimes for 10x or more of your original bet. If you're playing at a place that offers the 10x backup bet and the 20% loss rebate, you could do the same thing at craps.
The article implies that the 0.25 house edge was calculated before the rebates were considered, still leaving the ~0.08 house edge as a mystery. The quote below follows the quote from my original comment. The author of the article may have made a mistake, though.
When Johnson and the Trop finally agreed, he had whittled the house edge down to one-fourth of 1 percent, by his figuring. In effect, he was playing a 50-50 game against the house, and with the discount, he was risking only 80 cents of every dollar he played.
For example, his downside protection was 20% of a loss of at least $500k. If he was at -400k, he had no reason to quit; he should keep playing until he was ahead or at -500k.
Also, casinos previously negotiated 20% loss rebates on a LIFETIME basis. He negotiated the 20% loss rebate on a PER-TRIP basic, correctly pointing out that if he was ahead at a casino, he had no reason to return if another casino also offered the rebate.
The lt;dr summary is: He didn't cheat or count cards. The 20% loss rebate combined with blackjack being a nearly even game gave him an edge.
You could do the same thing at craps. At craps, there's an obscure rule that lets you make a bet at true odds after establishing a point, sometimes for 10x or more of your original bet. If you're playing at a place that offers the 10x backup bet and the 20% loss rebate, you could do the same thing at craps.