Peercoin's PoS implementation has nothing to do with NXT's PoS implementation.
PoS may be broken in one system, but perfectly safe in another.
Just like PoW may be susceptible to 51% attack and successful double spending and selfish mining in Bitcoin, but not susceptible to those attacks in Myriadcoin due to its 5 independant PoW algos.
Not to mention PPCoin/Peercoin is around 2 years old and was the first implementation of PoS ever. It was a trailblazer but it's not really relevant to discussing modern day PoS systems.