I think what they're trying to say is that some fraction of early births are because of something wrong with the fetus or maternal environment, and so there's a

I think what they're trying to say is that some fraction of early births are because of something wrong with the fetus or maternal environment, and so there's a selection effect where those who are born early are less healthy than those carried to full term. That is, you probably expect a random child removed at 28 weeks to survive a greater fraction of the time than those spontaneously born at 28 weeks (which already survive at a very high rate).