While it is true that the Azo dyes in Ciba/Ilfochrome are more stable than the Azomethine dyes in all chromogenic color materials, recent advances in coupler chemistry over the years, and the use of stabilzers in the coating have advanced both Endura and Crystal Archive papers such that they can reach up to 200 years storage with no significant fade. Although Fuji and Kodak differ about each others product, the fact is they were tested under different conditions by each company and when you boil it down, it is pretty much a wash.
How is it done? Well, the big problems with all dyes are UV and oxidation and so the coatings now contain UV absorbers and oxidation prevention chemicals. The former are like the UV filters used on cameras and the latter are similar to vitamin C and vitamin E. You cannot apply these methods to Ciba/Ilfochrome because couplers are in encapsulated droplets and the preservatives can be included. In fact, the solvent making these droplets aid in preservation. The Sulfonic acid based Azo dyes cannot be encapsulated easily.
Anyhow, there it is. Coupling chemistry caught up!
PE