The machines that contribute to a shared jackpot are called "progressives". They can be shared among machines in the same location, or among machines in a region. Gambling machines in the US use paytables that can be switched by the operator between levels of payback percentages, like 89%, 92%, 95%, etc. (Some poker machines are capable of returning greater than 100% payback, given perfect play by the user). When a jackpot is hit on a progressive, the new jackpot doesn't reset to 0, but has a built-in reset value, because who would want to jump on a machine with a jackpot of zero?
The largest jackpot on a progressive machine is only awarded if you are playing "max bet", meaning you are playing all lines at the maximum coin value. This couple probably wasn't. Most likely, they were playing all lines at some lower bet amount. The real problem is that the machine's manufacturer doesn't appear to have tested this scenario thoroughly. Casino operators are shielded from this by the usual disclaimer that any malfunction will void a win.
So, they might be able to sue the manufacturer, but not the operator. Probably neither.