Earlier quoted context omitted.
Parker is arguing that in many cases, the engineer can actually have a bigger impact at the bigger company (e.g. because it leverages economies of scale and having lots of startups fragmentizes the talent pool).
It's a self-serving argument. A bigger overall impact is more likely when there are tons and tons of smaller companies out there trying ideas (i.e. the chance of one of them hitting the next big thing). If all these engineers just go work at Facebook instead, Parker will feel the biggest impact in his stock value. Individual engineers might do better at FB than they would have failing at their own company, but on the…
What I'm saying is that, if you believe Parker's argument, it isn't necessarily against an engineer's best interests.