If they changed the Nether Horror functionality according to how they worded it… the Nether Horrors are not able to begin the initial 3 to 3.5 second (differs for some specs) cast that leads to the one-shot death channel when you cannot stop it via AoE CC (moving towards you or Velen).
If they are standing still at their spawn, and cast Nether Storm without moving, then they should not damage anyone. Their Nether Storm range is very small, and sometimes bears would Typhoon them away for a moment of safety.
The problem with Guardian challenge was that you had to hold off on popping Velen orbs when nether horrors just spawned, or they would finish their wind up before reaching Incapacitating Roar range.
This meant you popped orbs to
A. stall annihilate,
B. heal when healing is needed to last as long as possible,
C. Nether Horrors are either fully stacked up or did not recently spawn.
This change removes most of C, but given they can still spawn in final phase faster than Incap Roar’s cooldown, and you take a lot of unnecessary damage from Psytalon+melees, you should still aim to use the orb for all three conditions specifically including Psyfiends stacked up to quickly die before needing to incapacitate them.
If this is not the case, and the challenge becomes harder in any way, then that would mean some secondary, unintentional bug that should be reported.
Claims that they are “lying” or trying to “get players to buy wow tokens” are a little paranoid (when the market for consumables and gems = player controlled, with players not doing the challenges helping create the supply for players who are doing the challenges meaning some players uninterested in solo challenges get the opportunity to make gold).