This is a cool concept for a study, but in their methods and materials section, they didn't say anything about the general upbringing/living conditions of those 4 snakes.

There might be a massive difference between animals kept in dark breeder tubs vs. animals kept in vivariums with see-through doors. The former wouldn't even have the chance to focus their eyes on anything further away than the hind wall of the tub. I can't imagine that that wouldn't lead to some sort of atrophy.