Imagine you invested all of your retirement savings into "the index" a week before black Friday and then describe how the results are mathematically inevitable.
That's twisting some assumptions of the comparison. The comparison was to casinos, which do not bet decades of profits on a single day's gambling. The equivalent would be investing in your index every payday for decades, not suddenly moving it all at once. That's where the mathematical near inevitability comes from. You aren't betting on one day, one spin of the wheel, you're betting on a general trend across ten thousand spins/days.
As I'm sure you know, allocating your entire savings into a single asset at a single point in time is probably the worst possible strategy. Compare it to slowly shifting your savings allocation from cash to the same single asset gradually and continuously over the course of years. In the latter case, sure you get the Black Friday loss, but you also get the Golden Monday buying opportunity. Since nobody can reliably time the market, this is about as good as an unsophisticated person can do. And those who have enough money to pursue a more sophisticated strategy don't need a hacker news commenter telling them about allocation strategies.