to me, that text implies that the Tsaesci could not wear human armor, not that they had no sort of protective garb in their culture :shrug:
To be honest, I'm starting to doubt the accuracy of the 2920 books. First of all, katanas aren't designed to combat armor. Second, Akaviri shields existed not only in Oblivion but in Morrowind. Third, it's historical fiction.
Given that Reman's bodyguards had Akaviri blood and characteristics, I think it may be possible that the Tsaesci may have assimilated the Akaviri men and used them as soldiers in the invasion of Tamriel, only to lose most of them when Pale Pass was lost. This could account for the Tsaesci's unfamiliarity with the armor themselves. Afterwards, if they passed their variety of vampirism into Reman's guards, it would have given them both Akaviri eyes and vampiric longevity, both of which were noted.