Interesting story. And one that has me almost sympathising with the Russian Mafia.
Because anyone who finds a way to turn the tables on those devices is doing good work.
Those things are specifically designed to be as compelling as possible while they take every last bit of credit that the gambler has- providing compulsion wrapped up in the illusion of choice and autonomy. They bring to bear years worth of human-machine interaction design, countless rounds of A/B testing and our full knowledge of people's psychological flaws and biases- all to make the most efficient funnel of cash from the player to the operator. Some of the more advanced models seamlessly integrate ATM functionality into them, in order to stop anything getting in the way of more spins, and more losses for the gambler.
And if the process used was as described, it's difficult to see how it is any different from card counting- The player constantly calculates their odds, and only places a bet when the odds are in their favour. Card counting is absolutely not illegal, but will get you asked to leave, because it breaks the only real rule in the gambling industry- nobody is supposed to beat the house over the long run.
And as a technical aside, there is an easy fix to this sort of attack- never mind encryption of the data, that's really just security by obscurity. The way to get around "timing" spins on machine like this is simply to have the RNG cycle through the possibilities so fast, that it becomes impossible for human reaction times to land on a winning combination. Of course, doing that requires re-designing the chip in each machine, and physically upgrading each one.
Of course, that leaves an intriguing possibility that the article didn't cover. What if the machines that they're hitting are that previously went through Russia. With a glut of machines needing to be sold off after the 2009 ban, it's not inconceivable that some of these machines were (ahem) upgraded before being sold on to casinos throughout the world. Then, all you need to do is find where the cracked machines are, before cashing out at them.