The funny part is that this range is the range he had upon release. It did not last long tough. Nocturne received some heavy nerfs the patch after his release, including his ultimate range.
I agree tough, that nerf should be reverted. Maybe he was too strong back then, but it was in season 1. Let's just say the game changed quite a bit from that point (not really in favor of Nocturne).
Patch notes for context :
V1.0.0.115:
Duskbringer : - Attack damage bonus reduced to 15 / 25 / 35 / 45 / 55 from 20 / 30 / 40 / 50 / 60.
Unspeakable Horror : - Cast range reduced to 475 from 500.
- Leash range reduced to 550 from 600.
Paranoia : - Dash range reduced to 2000 / 2750 / 3500 from 2500 / 3250 / 4000.
Taken from lolwiki.