Be very careful about applying stereotypes like that to entire races. There are brutish and violent Altmer just as there are erudite and scholarly Nords. It's perfectly logical to assume that the Altmer would produce armour and weapons, due to their obvious utility on the battlefield.
I wasn't saying all Altmer are mages, i'm saying Altmer culture is majickally inclined. A majickally inclined culture is less likely to design their own style of armour, especially one like the Elven Armour from OB, as it would be cheaper and simpler to import armour made by other races in other provinences
And upon rereading Rislav the Rightous, i find it questionable that Leather was the only armour avaliable. It specifically says that at the time the book takes place, Indoril Nerevar and Dumac Dwarfking were both alive, meaning it was near the end of the Dwemers existence, and we know they have the ability to forge metal armour. We also know that since Skyrim is having civil problems, we can assume that at this time the Falmer are already extinct and that the Battle of the Snow Prince has already taken place, and we know that the Snow Prince had metal armour.