The explanation about PoSign isn't right in that article because he is saying that all nodes need to sign the block so it can be validated. If it work like that then anybody can sabotage XBY just by leaving one node offline. What really happens is that the nodes that are online at the time the transaction happen will sign the block and as i understood from Jhar the nodes that were offline will doublecheck the block when they come online but don't need to sign the block themselves.