The post itself provides ample evidence for why this is a rational position, especially for people with even mild risk aversion.
For instance, look at the table of "Startups that exited for more than they raised." Even among those who raised a Series E or later -- the most likely to exit -- only 40% experienced a favorable exit, and among those, the mean valuation growth was 2.26X.
Under optimistic assumptions, that's a "double your money" scenario with 40% odds, which is a not even a great bet in the aggregate -- but a pretty terrible bet for any individual.
Financial advisors counsel people all the time not to put their eggs in one basket. That's why index funds like VTI are so popular. They let you diversify your investments.
Heck, putting so many eggs into the basket of your employer is risky even after it goes public! I had a publicly traded employer who had a 401(k) match paid in company stock. Every pay period, I sold that stock and bought index funds instead.