Yes, I read it. As far as I can see, it talks about 'staking', so it doesn't address what happens when the tokens and 'expended' as opposed to 'staked'.
You have two competing chains: Chain 1 and chain 2. On chain 1, you spend a token on a good. On chain 2, you spend no token. You wait for chain 1 to win the race. You then stake on chain 2 to get your money back. If chain 2 then overtakes chain 1, you've executed a double-spend attack.
An equivalent attack wouldn't work on a PoW chain. If you do the equivalent of "staking" on chain 2, then you're computing hashes, which is costing real-life resources. In the PoS case, without slashing, staking on chain 2 is free. In fact, this is the rational move to make every time you spend a token; stake on competing chains to get your token back.
There was a PoS mechanism that makes people who cheat lose all their coins? I wonder if it is relevant here
... Aha, that's "slashing" -- the other members in the network would look at the two chains, and notice that you were misbehaving, and add transactions that remove parts of your coins? (They'd add to both chains? Or just the winning one?)
