Environmental factors can never truly be accounted for or dismissed, and we will never have a fully controlled study of people to where we can separate nature and nurture.
The common influences thing is mentioned specifically as a point of differentiation with other studies. They also state the following:
>While the present study has used a large population-based sample of Dutch twin youth (van Beijsterveldt et al. 2013), we recommend caution in generalizing our findings to other countries. An important next step therefore would be to replicate our findings in different populations.
Given their acknowledgment of common environmental influences, this is somewhat expected.