Ioulaum explains mind flayers and Netherese magic buuut Ioulaum was linked to Mystryl's weave not Shar's shadow weave. (I kinda wish the game wouldn't conflate shadow magic with Netherese magic. There was only one Netherese city that used Netherese Shadow magic)
The only Netherese city that used Netherese Shadow magic was also the only Netherese city active in recent times.
In-verse, it makes more sense that people would think of Netherese magic as shadow magic.
