I'm not sure how it is far fetched. A Curst maintains most of it's full faculties as opposed to being completely dead and reanimated nor did they seek out becoming one. I'm not sure why that realmshelp version had adjustments made, but the actual Monsters of Faerûn template states "Only about one in ten cursts retains its full, former Intelligence score—the rest have an Intelligence score of 8. Otherwise, cursts retain the ability scores they possessed in life."
They are essentially the same person they were when they died, aside from being technically undead and maybe a bit less able to reason. The CN assignment reflects the fundamental change in their behavioral characteristics while preserving the fact that they don't just go stark raving mad without any regard for anyone else.
It's all in the 3e alignment guidelines:
NE: does whatever one can get away with; no guilt, shame, remorse, apprehension about actions. (Socio/psychopath)
CN: follows his whims; values own liberty but doesn't strive to protect others' freedom. (Selfish)
NE: commits evil for its own sake; no regard for others.
CN: not motivated by good or evil; unpredictable, but not is not totally random behavior.
More importantly, CN believes that there is no order to anything, including their own actions (Nihilistic). From a psychological perspective, this would make sense of someone forced into an undying form having to live out unending days.