True. So, how do new Bitcoin nodes handle this issue?
Simple the longest chain is the best chain. The behavior is deterministic. It doesn't matter if a node is online, offline, or newly created, all nodes select the longest. There is only an issue where nodes select a chain based on criteria that can't be known to all nodes (i.e. chain A is shorter but it is the best chain because it came first and chain B is longer but came second and they differ by more than 720 blocks).
True.
New nodes would need to make sure they are on the legit chain.
But well, how do "Bitcoin" nodes? <<< I mean having a client looking like Bitcoin and feeling like Bitcoin does not necessarily mean it is the real Bitcoin network you are on, right?
Best is probably a better term than "legit" it is possible the best chain is the one created by an attacker (in both PoS & PoW) however the consensus system is limited to picking the best chain. The chain built from the genesis block which is the longest is the best chain. Any new node is relying on an assumption that other nodes are using the same selection criteria. If all nodes are using the same selection criteria (and it is deterministic and uniform across all nodes) they will all end up selecting the same chain as the "best" one.
It always comes down to that: costs. And I am with you: the costs for maintaining several PoS chains are significantly smaller than maintaining several PoW chains. However, I doubt 720 block to be sufficiently long enough to do any damage.
If it can be done once it can be done again. An attacker would be foolish to limit it to a single attack. If merchants are limited to waiting for 719 confirmations to ensure they aren't double spent then the attacker has done a good job of damaging the utility of the coin. The ability to fork offline clients is just an added bonus to add to the chaos. If your node goes offline you can't be assurance that you are on the same chain as other nodes.