Yep they are lucky. The stock market is a gamble in the short run. Even a home is a gamble sometimes. But to me what was important is that they started putting away a much larger share of their income; even negative returns are better than blowing your money on boats or SUV's or a timeshare or gold coins or alaska cruises or whatever stuff TV tells older people to buy. I think the main effect of the stock market doing well wasn't so much to give them returns as to make them super excited about investment; so they scrimped and saved a lot more because it was fun to see their numbers go up so fast. They also worked harder at their jobs (both are self employed) to make more money to put in their retirement fund.
It's also easy to shield yourself from massive losses through diversification. Vanguard even had someone talk them through selecting funds, even when they didn't have much money to invest. I also got them to pay off loans (cars, credit cards), which is the safest investment you can make for the returns.
I should also emphasize that, having started so late, stocks could never be their primary source of income. At a reasonable withdrawal rate I think they'll get about about $15-25k per year. Most of their income will still be from social security. But on a long enough time scale one of them will get senile or cancer or something, and we'll thank god for that big pile of money when it's time to hire help.