From the article, 19 / 1900 YC startups are above $1B, or 1% (not 0.2%). Many of the 1900 are young, so the fraction of any cohort that will eventually reach $1B is more than that.
The chance of having picked exactly AirBnB or Stripe from among YC startups is 2/1900, or 0.11% (not 0.002%). More companies out of that 1900 are likely to achieve similar valuations in the future.
The payoff ratio for angel investing is much higher than the 30:1 that Roulette pays. On the order of 1000:1 is typical ($10M cap safe - $20B IPO with 50% dilution on the way.)