The author's core point is that you are much more likely to become a senior engineer at a large company (and thus earn ~$250k in salary) than you are to hit the jackpot at a startup.
If you were to take the set of people who achieve senior tier (or comparable) at large tech companies and the set of people who earn at least $250k as a lump sum payout for working at a successful startup, you'd find that the former group is vastly larger than the latter.
If you were to do the same exercise and replace the second group with folks who actually got "rich" (for any reasonable definition) working for a startup, you'd find that it would not even be visible as a pixel next to the former group.
Statistically, you are simply better off trying to hit senior at large tech company if you want to optimize your career for wealth. This is so doable within five years that at large companies like Google and Microsoft you are expected to hit senior level, and if you don't it begins to reflect badly on your record. More explicitly, "senior" is the last level that everyone is expected to hit, and at which you are "allowed" to not seek further promotions.