I think they are saying you could 'roll back' on your own node to a point where you did have coin age, and try to attack from that point.
But it's not clear how you would get very far because there would be a longer chain soon and your coin age would be used up fast.
That is like saying you can't 51% attack a PoW network because the main chain is growing. Yeah if you have less computing power than the "good guys" you can't but what if you have more? With more than half of the network computing power you will eventually build the longest chain. With a PoS you will eventually build the longest chain if you have more than half of the network stake.
So on a hypothetical coin say in block 4,000 I (or a cartel of attackers) had more than half of the network stake. We sell these coins (exchange, p2p trades, spending, etc) and as of block 4,0001 the cartel has no coins however in block 4,000 we did. So despite the fact that the main chain is growing and may be ahead starting from block 4,000 and with over half of the network stake it is a mathematical inevitability that the attack chain will be longer. We double spend the sales in block 4,001 and eventually we will have a longer chain at which point it is published to the network, the network reorgs and we performed an attack with 0 cost and 0 risk.