Kind of hard to keep up with what exactly NXT is and whether it works or not:
As far as I know at least the first version of NXT's PoS is a direct clone of PPC's with some modifications, appeared lacking a good understanding of the security involved in PPC's PoS.
It isn't and never was:
After thinking about the mining algorithm I came to conclusion that original proof-of-stake used by PPC and NVC is a bit flawed. Bob could accumulate small amounts on different accounts during a long period of time and then attempt a 51% attack. Artificial limits like max 90 days don't seem to work as intended. Nxt will use a different proof-of-stake approach.
you would have to present a source code comparison between ppc and the first version of Nxt to make me think otherwise.