Yes, that's exactly right. The way we invest (a large number of small bets) means that it makes less sense for us to be conservative than it does for any other investor. So either JM is writing about other "incubators" or he's been misled by the fact that it takes a while for the world-changing companies to emerge as such. At year 2, Microsoft or Google or Facebook would not have seemed world-changing.
I wonder if this is simply because incubators of the YC variety are still such a new idea that we really don't know yet whether they'll produce a Google or Facebook yet. I doubt there's a deliberate focus on safe bets going on, since it seems to me it's in their best interest to take more risks if they're investing such a small amount at such an early stage.