Salamanders whose eyes are not fully functioning use less food than those with fully functioning eyes, which burn more energy to produce retinal chemicals and electrical discharges.
It only makes sense then, that over the few thousand years since the Noachian Flood, that the salamanders stranded who inherited defective eyes (ie -that burned less food) would survive longer than those with good eyes that used energy anyway.
The non working eyes are an excellent example of the "vestigial organs" which the Witchtower claims do not exist.
HB