He means if want to use a box defence or zone defence ,and if you want to have a good result with that ,all your players have to be good in OD and ID,because your player can defend against the PG,but the next minute he could be defending the Center.
Sometimes managers complain that 2- 3/3-2 and box does not work,but in most cases they don't have the players with the right skills to play a zone defence or box.
Primoss explanation is exactly what I think.