That part of the movie actually makes perfect sense to me.

In the real world, the vast majority of people are quick to misinterpret scientific findings to reinforce their own biases. Gattaca is the story of a world where the "Others" in that society are those who are not genetically engineered to culturally acceptable standards.

IMHO it is a very human and believable part of the story that most of society will simply choose to go along with being bigoted against the out group, and not spend the extra time and effort to figure out if their biases are actually correct. In the story, Anton tells Vincent his parents died assuming that he had died young, because the doctors told them he would.