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.
It is true there are different variations of PoS. None of them have been convincingly shown to be secure most especially not the versions used in 99% of the new PoS shitcoins that appear daily.