Hussar
Legend
When Disney bought Lucasfilm, they declared the Extended Universe (EU) material to be non-canon, and grouped it all under the Star Wars Legends label.
Certain aspects and elements of the EU have been re-introduced and made canonical over the intervening years, and cortosis looks to be the latest. IIRC, cortosis was introduced in the original Thrawn trilogy, one of the more popular EU sources.
I did read the Thrawn trilogy. What, thirty years ago? Something like that. Couldn’t tell you anything that stuck with me other than the kinda octopus looking thingies with a funny jam starting with “y” that were supposed to block Jedi powers.
Although thinking about it, the comment about the helmet blocking Sol’s attempt to read Dark Helmet’s mind makes some more sense now.