If one of your required qualifications is "be part of an existing program that old admits men" then the program is sexist merely by its requirements, even if those requirements are applied equally to all candidates. Neither does discrimination require malice or intent -- unintentional discrimination has exactly the same outcomes as intentional discrimination.
You also can't claim both that there were no women up-to-snuff therefore no discrimination existed, and also that selecting women would have imposed an additional burden on NASA. One of those scenarios claims there was no discrimination while the other one claims that discrimination was present and intentional but justified. And again, if you construct one of the requirements to be "must urinate via a penis" it doesn't matter that you apply that standard to everyone who applies, it's still sexist (and assumes that "male" is the default option and the females have "extra" requirements -- even if gender was a legitimate reason to discriminate, why are females the group selected against instead of males?).
But mostly this is absurd, because if NASA wasn't discriminating against females they would have been in the extreme minority of organizations at the time. Sexism was even more ridiculous then than it is now and to presume that NASA is or was somehow exempt from that culture-wide phenomenon just because they dreamed up an arbitrary set of performance standards is insane.