In that story about Nids vs Chaos the nids had no interest in warp entities as they are not made of any actual biomass. At first the hive mind ignored chaos. Then when it realized it was corrupting it's food source it went all out destroying it to the point that it ringed the entire equator in biovores, exocrines, and every other heavy artillery beast it could muster and slowly advanced on the point of origin of the demons bombarding them constantly until it nullified and wiped them out. Named greater deamons of all of the big 4 were there and they all got their asses handed to them in an embarrassing way.
Minor deamons and lesser deamons either winked out of real space or had trouble staying manifested because of the shadow in the warp. This wasn't a whole hive fleet. This was a small splinter that stopped off at a single world. It wasn't even part of leviathan which is by far the largest fleet to date and the one nobody has been able to stop even partially, all they could do was exterminatus a whole swath of systems to try and divert it into the orks.
See how big the area leviathan covers is compared to the eye of terror?
You might notice the hive fleets have arrived at the galaxy from different angles with leviathan popping up from below the galactic disk. If the fluff is moving forward maybe a new hive fleet will arrive either from the other side of the eye or between the eye and Terra.
The Nids are certainly one of if not the worst enemies chaos can face. Incorruptible, nonsustaining for them, and their mere existence more or less acts as the Cadian Spires because of the shadow.
I would love to see the nids show up and save the day only to become the worse threat.