The danger is in what gets defined as a "defect". I think we'd all be comfortable fixing congenital heart failure before birth, and I (hopefully) assume we'd all be very uncomfortable with "fixing autism". There's a big blurry area between those two things, and I think Trek has largely correctly tended on the conservative side.
But I like how this theory ties the timeline together in a way that makes a bit more sense, because the "risk of another Khan" is not, at this point, the most interesting problem with gene editing in Star Trek to me
If you want to make the argument that people should be able to modify themselves I 100% back that up. In fact I think that probably would have fixed my problems with this episode, if this is something more like a rite of passage that Ilyrian adolescents choose to undergo then the whole thing gets way less ethically messy because now you're letting people make decisions about themselves that are socially influenced instead of having decisions made for them that are socially enforced. But I have really strong aversions to society deciding what types of bodies and minds even get to be born.