The point is it mitigates the on-chain attack surface which is still prevalent on PoW. Off-chain attacks are still possible for all consensus mechanism.
What on-chain attack surface? The only way to permanently knock a PoW chain offline is to consistently out-mine everyone else. In PoS, once you lose BFT quorum (1/3 of all votes), it's game over.
No distributed system is guaranteed to make forward progress if over 1/3 of its voting nodes is faulty, full stop. Once an adaptive adversary controls more than 1/3 of the voting power, they can forever delay the remaining 2/3 of the voting nodes from reaching consensus. Hell, they'd even be able to delay votes from other nodes to slash their stolen stash.
From this, I can conclude at least one of the following: