I guess there's a little bit of a misunderstanding here.
The maternal womb is also considered an environmental factor, so by the time they are born the offspring has already been affected, greatly, by the environment. And that's precisely one of the major hypotheses of this paper.
Of course, genetics also influences our personalities greatly, but as seen in this paper with genetically identical fish; even in this case there is shown to be individuality. Now, this paper doesn't say anything about how much of one's personality is genetics or how much is influenced by the environment. What it says is that in genetically identical individuals that grow in (pretty much) identical conditions there is already individuality that's present at the time of birth which is possibly mediated by the pre-natal environment.