In this case the DB9 connector is not just surface-mounted, as it sits on the board edge so the force when inserting it will be taken by the board, and the surface-mounted solder points are at both sides of the board so the pull force will not cause bending as it would in a plain surface-mounted connector.

My hunch is that all that makes up for it. I would be very surprised if it breaks off. I’m however not an expert beyond having repaired several surface-mounted connectors that broke off in my own computers.