No this isn't true, they come from a stupidly low drop rate via farming the Wood, Cloth and Ore nodes in only Upper Craglorn... Or about 10K gold on Guild Stores.

I never found a single one via plant nodes and I've been farming every single day since Upper Craglorn came out. I've also never found one from the usual trait stone places (Nightstands/Chests/Cabinets) in Upper Craglorn, and they do not come via Hirelings either. The drop rate via Ore/Wood/Cloth nodes hasn't changed that I'm aware of other than the tweak to Survey nodes a week after Writs came out. But since ZoS hasn't ever been transparent with anything about the game really, who knows, Nirncrux might drop from Fishing nodes at a very small rate... Oh, they also come from Heavy Sacks in Upper Craglorn very rarely.