> You honestly believe that any one of those companies, none of which has been around for more than ten years, could reach a market cap of $200B within 5 years?

That's not the bet. The bet is that they will, in aggregate be worth $200B.

    "Proposition 1: On January 1st, 2020, these companies 
    will be worth at least $200B in aggregate."

Edit: Can't read. The parent is in response to the grandparent, not the bet.