I don't think it ever needed to be addressed. Given 20+ billion human beings around the inception of the Spartan II project, a number of individuals with the requisite age, intelligence, and athletic ability are bound to be present.

One of my pet peeves about Spartan fans is that they keep talking about the Spartan's 'superior genetics' or 'perfect genes'. Superior how? Perfect how? The biggest selection criterion for the candidates would have been "Does this individual have the physiology needed to survive the augmentation process and use the upgrades we give them?"

Genetics are just raw material. Without the Spartan II program, the Spartan candidates may have become the next Otto von Bismarck, but they might also have squandered that potential as drifters or social workers. Halsey and her cohorts stole the Spartan's free agency, but in return those Spartans were leveraged to their ultimate potential, made more human than human.

And, I guess that's what I dislike about the Librarian's Geas. It removes that conflict entirely by removing everyone's agency. Nobody ponders the morality of marionettes.