This is not true. Similar to PoW
That's a false statement. Proof-of-Stake isn't similar to Proof-of-Work because it's cost-free. Unlike Proof-of-Work, you can retry attack attempts multiple times using the same coins.
thats subject to a 51% attack, the PoS will need to hold 51% coins in order to be able to attack the coin. But holding 51% and then attacking your own coin normally is not in your interests.
You
don't need 51% of coins to perform a successful attack. You only need less than 15% of
active weight here.
ATTEMPTS = 1 / STAKE ^ CONFIRMATIONS
If you have 25% of
active weight then you need ~256 attempts;
If you have 15% of
active weight then you need ~1975 attempts;
If you have 10% of
active weight then you need ~10000 attempts.
Note that active weight has no direct relation with total supply. It's calculated as (volume * timeweight) for coins participating in the network protection. E.g. if there are only 50% of coins @ maximum timeweight used for stake generation, then you need ~7.5% of total coins volume in order to perform double-spend attack for the reasonable time.