That is indeed the average amount invested by YC. As stated below in recent batches there has been the startfund blanket investment which used to be $150k and is now $80k. Technically speaking this blanket investment has nothing to do with YC and is given as uncapped convertible note (basically the best possible terms).
To address if this is enough, I'd say you'd be surprised how much you can make things stretch if you absolutely have to, but most companies also raise a small amount of money after YC's demo day as well to help them continue. You can think of YC as a startup bootcamp at the end of which you have to prove yourself to get the opportunity (further investment, usually) to continue on. With the startfund though, you get more than those three months to do so.
In short, yes, YC invests $20k and produces $6.1bn in value across the alumni.
To address if this is enough, I'd say you'd be surprised how much you can make things stretch if you absolutely have to, but most companies also raise a small amount of money after YC's demo day as well to help them continue. You can think of YC as a startup bootcamp at the end of which you have to prove yourself to get the opportunity (further investment, usually) to continue on. With the startfund though, you get more than those three months to do so.
In short, yes, YC invests $20k and produces $6.1bn in value across the alumni.