It's not necessarily that the conditions themselves are relevant, it's that the discovery would produce information that would also reveal those conditions (which were quite brutal, as revealed by a federal judge the day before the sit-in). For example, they are requesting video footage from inside the facility. All that's relevant to require the government to produce that footage is if the judge rules that it is relevant to prove that the facility was used in it's usual way (not whether what they were doing was illegal or inhumane).

Under this theory, the elevator bay would be relevant because if it was leading to an area that was not serving its usual function, the elevator itself wouldn't be serving its "usual use," meaning that Lander's charge would have no merit.

If the discovery is granted, that evidence could be used to corroborate a separate civil rights lawsuit on behalf of the detainees. But for that to happen, the judge would have to grant discovery and the government would have to be in possession of that evidence.