"eugenics" in its mild form means not having defective children. If a couple discovers that they cannot avoid a birth defect in a child, that is, a 100% certainty, perhaps after having one, then according to the quotes you cite, they could openly state that they will refrain from intercourse at fertile times so as not to beget another defective child.
Since no child is conceived, no innocent person is harmed.
Should they miscalculate their ovulation and become pregnant anyway, they would just have to hope for the best and love the second child.
The fact that the Pope may have permitted "eugenics" in some mild form as this does not mean he permits it in the "blue-eyed genius-athlete" form.