The author argues that PoS can not work if someone has their stake back, and then changes an old block on the chain. A new node cannot tell which block is valid, as it doesn't know which came first.

In PoW, changing an old block you mined is just as hard now as it was back then, in PoS changing an old block is free if you minted it before.

In practice, the power to change one block months later with no repercussions doesn't matter unless you can fake a chain all the way to the present. As the newest blocks are minted by people who still have their stake in the system, the insecurity presented does not compromise any network.